Add test coverage for native format flink upgrades (minor versions)

Check test coverage for:

Flink minor (1.x → 1.y) version upgrade

Parametrised SavepointMigrationTestBase to work on:

canonical savepoints

native savepoints

aligned (retained) checkpoints

Assignee should contact release managers and decided whether to already create 1.15 (snapshot) artefacts for native savepoints and aligned checkpoints. If we do so, we might make release manager work more complicated. However if we don’t, the test will be a dead code until release manager creates those artefacts, which can also make the release manager work more difficult if test explodes during RC creation.