Update release process to include a rc and testing of upgrade deployment repo prior to release.

Description

Build upon https://microsoft.github.io/AzureTRE/latest/tre-developers/release/

To ensure deployments and upgrades are smooth after a release, I suggest we create a release candidate first, ensuring extended tests complete on the release PR. Create a branch in the deployment repo with any updates. Test that on a fresh deployment and upgrade from the prior version. Once successful, verify docs are correct as far as secrets etc, create a release (not rc), and PR the new deployment branch.

consider before making a release

• Remove any Terraform or Database migrations that are from previous versions.
• Do an upgrade test from the last version, and not skipped any versions.
• Update terraform providers to latest versions

Start with the release process documentation at microsoft.github.io/AzureTRE/latest/tre-developers/release/ and review how the release PR and deployment repository are currently handled. Define the release-candidate, fresh-deployment, and upgrade tests, including secrets and migration checks; done means the upgrade from the prior version succeeds, the documentation is verified, and the final deployment branch is ready for a PR.