fix(gax): drop jsr305 annotations left over after syncing with main

[macastelaz]

Why

After the main sync (03af2000542), the dependencies check fails on every PR targeting this branch, for example #14559:

[ERROR] Used undeclared dependencies found:
[ERROR]    com.google.code.findbugs:jsr305:jar:3.0.2:compile
... gax-grpc / gax-httpjson: Dependency problems found

main moved GAX off jsr305: it uses JSpecify for nullness, and gax-grpc/gax-httpjson no longer declare jsr305. Two javax.annotation uses that came in with #13995 (certificate rotation retries) are still on this branch. They compile, because jsr305 still arrives transitively, but dependency:analyze correctly flags them.

What changes

These are annotation-only edits with no behavior change:

• gax-grpc/.../ChannelPool.java: the @GuardedBy("entryWriteLock") annotation (from javax.annotation.concurrent) becomes a // Guarded by entryWriteLock. comment. I didn't switch to Error Prone's @GuardedBy, because gax-grpc doesn't depend on Error Prone annotations, so that would just trade one undeclared dependency for another. On main, gax-grpc uses no @GuardedBy.
• gax-httpjson/.../RefreshingHttpJsonChannelTest.java: javax.annotation.Nullable becomes org.jspecify.annotations.Nullable, as in the rest of GAX.

These are the only javax.annotation imports in gax-java, google-auth-library-java and java-core that are on this branch but not on main.

Tests

dependency:analyze was run the same way as .kokoro/dependencies.sh (mvn dependency:analyze -Pquick-build -DfailOnWarning=true -Dmdep.analyze.skip=false) on gax-grpc and gax-httpjson:

• without this change: Used undeclared dependencies found: com.google.code.findbugs:jsr305, BUILD FAILURE (the same as CI);
• with this change: BUILD SUCCESS.

Unit tests: gax-grpc 252 and gax-httpjson 234 tests, 0 failures. fmt is clean.

[gemini-code-assist]

Code Review

This pull request replaces the javax.annotation.concurrent.GuardedBy annotation with a comment in ChannelPool.java and updates RefreshingHttpJsonChannelTest.java to use org.jspecify.annotations.Nullable instead of javax.annotation.Nullable. There are no review comments, and I have no additional feedback to provide.