Update protobuf_grpc

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
io.grpc:protoc-gen-grpc-java	1.68.1 -> 1.68.2
com.google.protobuf:protoc (source)	4.28.3 -> 4.29.1
io.grpc:grpc-bom	1.68.1 -> 1.68.2
com.google.protobuf:protobuf-bom (source)	4.28.3 -> 4.29.1

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Release Notes

grpc/grpc-java (io.grpc:protoc-gen-grpc-java)

v1.68.2

Bug Fixes

• api: When forwarding from Listener onAddresses to Listener2 continue to use onResult (https://github.com/grpc/grpc-java/pull/11688). This fixes a 1.68.1 "IllegalStateException: Not called from the SynchronizationContext" regression (#​11662) that could be seen in certain custom NameResolvers
• okhttp: If the frame handler thread is null do not schedule it on the executor (https://github.com/grpc/grpc-java/pull/11716). This fixes a 1.68.1 NullPointerException regression when a custom transportExecutor was provided to the channel and it did not have enough threads to run new tasks

Improvements

• examples: Use xds-enabled server and xds credentials in example-gcp-csm-observability (https://github.com/grpc/grpc-java/pull/11707)

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