[BUG] Fix missing javadocs error for nested classes generated using protobuf

[VachaShah]

Describe the bug

Currently, the classes generated using protobuf are annotated with Generated and ignored from the missing javadoc gradle task (Done in #7604). However, the annotate_code in gradle task does not add the Generated annotation to nested classes. This is causing the missing javadoc to fail due to nested classes. For example: https://github.com/opensearch-project/OpenSearch/actions/runs/7561172941/job/20588856740?pr=11910 for proto message: https://github.com/VachaShah/OpenSearch/blob/search-protobuf-part1/server/src/main/proto/server/search/QuerySearchResultProto.proto#L36.

Current workaround is to add the classes to javadocMissingIgnore.

Related component

Other

To Reproduce

1. Created a nested proto message and generate Java code.
2. Run ./gradlew :server:missingJavadoc.
3. See error for missing javadoc for nested classes/interfaces.

Expected behavior

No missing javadocs error for proto generated nested code.

Additional Details

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Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
protocolbuffers/protobuf#42 (comment)

[peternied]

[Triage - attendees 1 2]
@VachaShah Thanks for filing.