doc: Readme documentation and links are not up-to-date

documentation and java doc links on README page is not up-to-date.

For 5.x latests, release please was able to pick up first occurrence of version, not others.
For 3.x & 4.x branches, manual updates needed on each release.

https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/adb57c6165a384b0236d8599598f8c699fccc6d3/README.adoc?plain=1#L13-L17

Start with README.adoc lines 13–17 and inspect the referenced documentation and JavaDoc links. Check how Release Please handles version occurrences for 5.x and how 3.x and 4.x branch releases are updated; done means the README links and version references are current across the affected branches.

Written by the indexing model from the issue text.