Plugins: Add backcompat for sha1 checksums

[rjernst]

With 6.0 rc1 we now publish sha512 checksums for official plugins.
However, in order to ease the pain for plugin authors, this commit adds
backcompat to still allow sha1 checksums. Also added tests for
checksums.

closes #26746

[s1monw]

LGTM awesome that we now have tests!!

[mgreau]

@rjernst @jasontedor As far as I understand the code, we will print a warning message for the community plugins uploaded on Maven Central (Please update the plugin to use a sha512 checksum) but the authors can't do it until that Maven Central (Sonatype Nexus) supports it or generates those sha512 files. Should we do an exception (for the warning message) until Maven Central supports it?

[mgreau]

@rjernst @jasontedor I have asked to Sonatype to remove the security level on the JIRA issue, they did it very quickly so you should be able to view it now. By the way, I had the confirmation that sha512 is not supported by Maven Central yet, by this person:

That's correct. We currently only publish md5 and sha1 checksums. I've started asking around here on the feasibility of generating and publishing sha512.