Update release checklist (various process tweaks)

Update release checklist (various process tweaks)

Ensure the release checklist reflects our current release process:

  - add an additional deadline for introducing code changes ("code
    freeze"); only test and documentation tweaks can be applied to
    pending releases after this deadline passes,

  - notify Support and Marketing about an impending release earlier in
    the process so that they have time to schedule a release note review
    before the tagging deadline,

  - examine current test results on all platforms in advance, to prevent
    diagnosing and addressing test failures in the last minute before
    the tagging deadline,

  - check Perflab results earlier in the process to leave some room for
    addressing any potential problems before code freeze,

  - ensure empty release notes for the next set of releases are prepared
    after public release.