Add migration docs for 8.0

Author: ywelsch

docs/reference/migration/migrate_8_0.asciidoc

@@ -12,6 +12,7 @@ See also <<release-highlights>> and <<es-release-notes>>.
 coming[8.0.0]
 
 * <<breaking_80_analysis_changes>>
+* <<breaking_80_discovery_changes>>
 * <<breaking_80_mappings_changes>>
 
 [float]
@@ -31,4 +32,5 @@ Elasticsearch 7.x in order to be readable by Elasticsearch 8.x.
 =========================================
 
 include::migrate_8_0/analysis.asciidoc[]
+include::migrate_8_0/discovery.asciidoc[]
 include::migrate_8_0/mappings.asciidoc[]

docs/reference/migration/migrate_8_0/discovery.asciidoc

@@ -0,0 +1,29 @@
+[float]
+[[breaking_80_discovery_changes]]
+=== Discovery changes
+
+[float]
+==== Removal of old discovery settings
+
+All settings under the `discovery.zen` namespace, which existed only for BWC reasons in 7.x,
+will no longer be supported. In particular, this includes:
+
+- `discovery.zen.no_master_block`
+- `discovery.zen.hosts_provider`
+- `discovery.zen.ping.unicast.concurrent_connects`
+- `discovery.zen.ping.unicast.hosts.resolve_timeout`
+- `discovery.zen.ping.unicast.hosts`
+- `discovery.zen.unsafe_rolling_upgrades_enabled`
+- `discovery.zen.commit_timeout`
+- `discovery.zen.fd.connect_on_network_disconnect`
+- `discovery.zen.fd.ping_interval`
+- `discovery.zen.fd.ping_timeout`
+- `discovery.zen.fd.ping_retries`
+- `discovery.zen.fd.register_connection_listener`
+- `discovery.zen.join_retry_attempts`
+- `discovery.zen.join_retry_delay`
+- `discovery.zen.max_pings_from_another_master`
+- `discovery.zen.send_leave_request`
+- `discovery.zen.master_election.wait_for_joins_timeout`
+- `discovery.zen.master_election.ignore_non_master_pings`
+- `discovery.zen.publish.max_pending_cluster_states`

docs/reference/modules/discovery/discovery-settings.asciidoc

@@ -8,22 +8,14 @@ Discovery and cluster formation are affected by the following settings:
     Provides a list of master-eligible nodes in the cluster. Each value has the
     format `host:port` or `host`, where `port` defaults to the setting
     `transport.profiles.default.port`. Note that IPv6 hosts must be bracketed.
-    The default value is `["127.0.0.1", "[::1]"]`. See <<unicast.hosts>>. This
-    setting was previously known as `discovery.zen.ping.unicast.hosts`. Its old
-    name is deprecated but continues to work in order to preserve backwards
-    compatibility. Support for the old name will be removed in a future
-    version.
+    The default value is `["127.0.0.1", "[::1]"]`. See <<unicast.hosts>>.
 
 `discovery.seed_providers`::
 
     Specifies which types of <<built-in-hosts-providers,seed hosts provider>>
     to use to obtain the addresses of the seed nodes used to start the
     discovery process. By default, it is the
-    <<settings-based-hosts-provider,settings-based seed hosts provider>>.  This
-    setting was previously known as `discovery.zen.hosts_provider`. Its old
-    name is deprecated but continues to work in order to preserve backwards
-    compatibility. Support for the old name will be removed in a future
-    version.
+    <<settings-based-hosts-provider,settings-based seed hosts provider>>.
 
 `discovery.cluster_formation_warning_timeout`::
 
@@ -55,20 +47,12 @@ Discovery and cluster formation are affected by the following settings:
 `discovery.seed_resolver.max_concurrent_resolvers`::
 
     Specifies how many concurrent DNS lookups to perform when resolving the
-    addresses of seed nodes. Defaults to `10`. This setting was previously
-    known as `discovery.zen.ping.unicast.concurrent_connects`. Its old name is
-    deprecated but continues to work in order to preserve backwards
-    compatibility. Support for the old name will be removed in a future
-    version.
+    addresses of seed nodes. Defaults to `10`.
 
 `discovery.seed_resolver.timeout`::
 
     Specifies how long to wait for each DNS lookup performed when resolving the
-    addresses of seed nodes. Defaults to `5s`. This setting was previously
-    known as `discovery.zen.ping.unicast.hosts.resolve_timeout`. Its old name
-    is deprecated but continues to work in order to preserve backwards
-    compatibility. Support for the old name will be removed in a future
-    version.
+    addresses of seed nodes. Defaults to `5s`.
 
 `cluster.auto_shrink_voting_configuration`::
 
@@ -192,7 +176,4 @@ APIs are not be blocked and can run on any available node.
 * For the cluster to be fully operational, it must have an active master.
 ===============================
 
-WARNING: This setting replaces the `discovery.zen.no_master_block` setting in
-earlier versions. The `discovery.zen.no_master_block` setting is ignored.
-
 --