release-25.1: mixedversion: add MinBootstrapVersion option

[blathers-crl]

Backport 2/2 commits from #142211 on behalf of @DarrylWong.

/cc @cockroachdb/release

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This change allows test writers to set a MinBootstrapVersion. This option will tell to the mixed version framework to not generate any plans that create a cluster on any versions older than MinBootstrapVersion.

With this new option, the http-register-routes/mixed-versions test now sets the MinBootstrapVersion to 24.2, the oldest release that has the fix contained in #138342.

Fixes: #138861
Release note: none

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Closes: #139410
Release justification: Test only change

[blathers-crl]

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[blathers-crl]

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[cockroach-teamcity]

This change is 

[DarrylWong]

Cherry picked #143661 on top