gradle deprecation fixes testing

[arunvenmany-ibm]

Creating a PR to test builds with --warning-mode=all and fixing issues

[arunvenmany-ibm]

Hi @cherylking

I have fixed most of the issues. There are more issues, but all of them are on the test builds of springboot 2. I suspect gradle 9 doesnt support springboot 2. Same tests are not showing any warning if I change spring boot version to 3

Update:- I went through spring boot gradle plugin documents and found the following
Please find the index of documents for different versions of spring boot :- https://docs.spring.io/spring-boot/docs/

• Spring boot 2 version we use in our builds is 2.6.7
  As per document, it should be used with gradle 7.x only
• Last version of spring boot 2 is 2.7.18
  As per document, maximum supported gradle version is 8.x
• first version of spring boot 3 is 3.0.0
  As per document, it supports 7.5 or higher
• Spring boot 3 versions we use in our test builds is 3.1.3
  As per document, it supports 7.5 or 8.x, but this should be supporting gradle 9, as its not throwing and deprecated warnings

Do we need to keep supporting spring boot 2, or should we mention it in release notes that "from gradle 9, spring boot gradle plugin support is removed for version 2.7.18 or below"

[cherylking]

Do we need to keep supporting spring boot 2, or should we mention it in release notes that "from gradle 9, spring boot gradle plugin support is removed for version 2.7.18 or below"

@arunvenmany-ibm As discussed in scrum, we should wait until Gradle 9 has been released and then see if the springboot plugin gets updated to support Gradle 9. We should keep supporting springboot 2 if possible.

[cherylking]

Just noting here that we decided any breaking changes (like the change to use the Java extension to get the source/targetCompatibility configuration) would need to be made in a separate PR and would require a bump in the major version of our plugin. This will be deferred until Gradle 9 is actually released and we can test with it and see if we can support both ways to configure those properties.