Rename nightly_6_1 params to nightly_next (grpc#50)

Rename nightly_6_1 params to nightly_next; see
apple/swift-nio#3122

Author: rnro

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
       linux_5_9_enabled: false
       linux_5_10_enabled: false
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
-      linux_nightly_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
 
   construct-plugin-tests-matrix:

.github/workflows/pull_request.yml

@@ -23,7 +23,7 @@ jobs:
       linux_5_9_enabled: false
       linux_5_10_enabled: false
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
-      linux_nightly_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
 
   construct-plugin-tests-matrix: