fix(deps): Update the Java code generator (gapic-generator-java) to 2.31.0 (#1502)

* fix(deps): Update the Java code generator (gapic-generator-java) to 2.31.0

PiperOrigin-RevId: 596645164

Source-Link: googleapis/googleapis@4a0e62e

Source-Link: https://github.com/googleapis/googleapis-gen/commit/a10ed6a77676c37e60799098d48d0afb16008613
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiYTEwZWQ2YTc3Njc2YzM3ZTYwNzk5MDk4ZDQ4ZDBhZmIxNjAwODYxMyJ9

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* fix javadoc html manually

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Alice Li <[email protected]>

Author: gcf-owl-bot[bot]