3.0.x(main) to 2.1.x

[jvalkeal]

Originally 3.x (in main branch) was created to rework codebase on Spring Boot 2.6.x. After doing first milestone 3.0.0-M1 we realised that it'd be confusing to create a new major version 4.x just to update Spring Boot to 3.x. In future to align Shell/Boot versions we're doing this:

• Switch versions in main branch from 3.0.0-SNAPSHOT to 2.1.0-SNAPSHOT
• Rename 3.0.0-M1 milestone to 2.1.0-M1 and put a note linking to this issue
• With next milestone release 2.1.0-M2, describe what has happened.
• Remove 3.x related artifacts from spring artifactory
• Rename git tag

[jvalkeal]

3.0.0-M1 artifacts removed from spring repos