Set minimum python version to 3.12 in CMake

[jclarkeSTFC]

Also updated copilot instructions and codebase analysis to reflect this change.

Hopefully this sorts out recent CI problems.

Here the ${_res_py_content} variable is empty, presumable because the command above isn't working, which I think is due to PYRCC5_CMD being incorrect, which may be due to the wrong min Python version in the cmake.

To test:

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Reviewer

Your comments will be used as part of the gatekeeper process. Comment clearly on what you have checked and tested during your review. Provide an audit trail for any changes requested.

As per the review guidelines:

• Is the code of an acceptable quality? (Code standards/GUI standards)
• Has a thorough functional test been performed? Do the changes handle unexpected input/situations?
• Are appropriately scoped unit and/or system tests provided?
• Do the release notes conform to the guidelines and describe the changes appropriately?
• Has the relevant (user and developer) documentation been added/updated?
• If the PR author isn’t in the mantid-developers or mantid-contributors teams, add a review comment rerun ci to authorize/rerun the CI

Gatekeeper

As per the gatekeeping guidelines:

• Has a thorough first line review been conducted, including functional testing?
• At a high-level, is the code quality sufficient?
• Are the base, milestone and labels correct?

[github-actions]

Unit test results

2 861 tests   2 861 ✅  12h 41m 18s ⏱️
    1 suites      0 💤
    1 files        0 ❌

Results for commit 3956293.

[github-actions]

System test results

808 tests   792 ✅  3h 3m 40s ⏱️
 11 suites   16 💤
 11 files      0 ❌

Results for commit 3956293.