nightly job should do API compatibility report

we should add running the API compatibility check (dev-support/checkcompatibility.py) to our nightly tests.

• in master check against head of prior major release
• in major release branch (e.g. branch-2) check against head of prior minor release
• in minor release branch (e.g. branch-2.0) check against prior maintenance release
• update release docs to suggest RM update above after making a release.