docs: update compatibility table

[breun]

This pull request adds the recently released Spring Boot 3.3.x to the compatibility table. Just like Spring Boot 3.2.x, Spring Boot 3.3.x is compatible with Spring Cloud 2023.0.x and Spring Framework 6.1.x.

I've also included these minor improvements in this PR:

• More precise Spring Framework compatibility for Spring Cloud GCP 4.x: 6.x → 6.0.x
• Spring Cloud GCP 2.0.x → 2.x, for consistency with the other $MAJOR.x entries
• Added links for all versions

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[burkedavison]

Associated Spring Boot 3.3.x compatibility PR: https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/2990

[burkedavison]

This commit was merged into https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/2990 which has now been merged into main.

Thank you @breun for the improvements!