Update reference, MiMa previous version and sync TASTy version

[WojciechMazur]

• Update reference version to 3.6.3-RC1 (from 3.6.0)
• Update mima previous binary verison to 3.6.2 (instead of unofficial 3.6.1)
• Set TASTy version to 28.7-experimental-1 - it should have been set when branching of 3.6.3.
• We now document better how and when tasty version should be set
• Add additional runtime test to ensure we don't emit invalid TASTy version during Release / NIGHTLY releases and the expected version set in build matches version defined in TastyFormat - resolves Verify TASTy versions before releases #13447

Reference version and TASTy version bump needs to be performed at the same time (otherwise usage of compiler with stable TASTY would lead to failure in from-tasty non-bootstrapped tests)

[test_non_bootstrapped]

[WojciechMazur]

This test actually would be never executed - RELEASEBUILD and NIGHTLYBUILD are set only for the release but not in tests executed as a requirement to start releasing job

[bishabosha]

glad there's finally a real test :)