message: Add v3 development specific info to PR template
diff: Closes SAP/cloud-sdk-backlog#ISSUENUMBER.
  +- [ ] I know which base branch I chose for this PR, as the default branch is `v2-main` now, which is not for v3 development.
  +- [ ] If my change will be merged into the `main` branch (for v3), I've updated (V3-Upgrade-Guide.md)[./V3-Upgrade-Guide.md] in case my change has any implications for users updating to SDK v3
  +
  <!-- Check List:
  * Tests created/adjusted for your changes.
  * PR title adheres to [conventional commit guidelines](https://www.conventionalcommits.org).
  