Fix events javadoc references

[seregamorph]

Description

Current javadoc for events refer to https://developer.github.com/v3/activity/events/types which redirects to https://developer.github.com/webhooks/event-payloads/ which declares it's obsolete, go to https://docs.github.com/en/developers/webhooks-and-events/webhook-events-and-payloads
The javadoc references for GHEventPayload subtypes fixed accordingly, extended with additional links mentioned in GitHub documentation.

Before submitting a PR:

We love getting PRs, but we hate asking people for the same basic changes every time.

• Push your changes to a branch other than master. Create your PR from that branch.
• Add JavaDocs and other comments
• Write tests that run and pass in CI. See CONTRIBUTING.md for details on how to capture snapshot data.
• Run mvn clean compile locally. This may reformat your code, commit those changes.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.

When creating a PR:

• Fill in the "Description" above.
• Enable "Allow edits from maintainers".