Salsa-CI: Re-enable upgrade tests for Buster to Sid
Upgrade tests from Stretch to Sid and Buster to Sid were previously disabled due to Bug#993755. Re-enable upgrade tests from Buster now with workaround applied in the step that enables Sid repositories.
Do not re-enable any Stretch upgrade tests - that is already oldoldstable and not even the most outdated Debian LTS users should attempt to jump from Stretch MariaDB 10.1 directly to Sid/Bookworm MariaDB 10.6.
Also refactor the structure so that we have:
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Simple upgrade tests to ensure the newly built packages upgrade nicely replacing the previous packages.
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Complex upgrade tests to ensure the newly built packages upgrade nicely and that the distro release upgrade also works.
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Cross-vendor upgrade tests to ensure the newly built packages upgrade nicely replacing similar but not identical packages from another third party repositories.