Prefer-lowest build uses installer version that may not be the lowest that module supports

[GuySartorelli]

Prefer-lowest builds install the version of installer that all the other CI runs use, which may not respect the actual dependencies for the module.
This can result in problems we aren't catching when we forget to bump dependencies.

Acceptance criteria

• When CI runs include an installer version being added by the matrix, the version of installer used in the prefer-lowest build is the lowest version allowed in the dependencies for that module

PRs

• NEW Start with lowest installer version for --prefer-lowest #97
• ENH Increment the version of installer for --prefer-lowest builds gha-ci#130
• ENH Use the lowest possible version of installer for --prefer-lowest builds gha-ci#131

[emteknetnz]

Example run https://github.com/emteknetnz/silverstripe-elemental/actions/runs/9752302859

Composer bit looks like this running on the 5 branch for the --prefer-lowest build:

All other jobs just use installer 5.x-dev

[GuySartorelli]

PR merged and new version tagged.
Reassigning to Steve to double check it works as expected

[emteknetnz]

It's doing what it should