Add oldrel R CMD check

[Bisaloo]

There is a gap in our coverage where we jump directly from oldrel-1 to release.

This is also a problem because oldrel is defined as a required test to merge PRs. Since this test doesn't exist, PRs never reach the stage where they can be merged (excepted if an admin bypasses the test requirements).

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[Bisaloo]

Additionally, the required tests use ubuntu-20.04 when the new tests use ubuntu-latest.