[WIP] Default to windows-2025 community gallery images

[mboersma]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Changes the "default Windows image" logic to use windows-2025 containers, instead of the deprecated windows-2019 versions.

Which issue(s) this PR fixes:

Fixes #5357

Special notes for your reviewer:

This is unlikely to "just work," so I'm using this PR to gather debug information.

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

Default to windows-2025 community gallery images

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[k8s-ci-robot]

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[mboersma]

/test pull-cluster-api-provider-azure-e2e

[mboersma]

There's also the question of how to keep the upgrade tests from failing when an earlier codebase provisions windows-2019 nodes and the upgraded CAPZ finds the same version, but from windows-2025. (See #5266 for more context.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.27%. Comparing base (33eeeee) to head (a105ddf).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5635   +/-   ##
=======================================
  Coverage   53.27%   53.27%           
=======================================
  Files         272      272           
  Lines       29522    29522           
=======================================
+ Hits        15727    15729    +2     
+ Misses      12980    12978    -2     
  Partials      815      815           

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[willie-yao]

/retest

[k8s-ci-robot]

@mboersma: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-azure-conformance-custom-builds	a105ddf	link	false	/test pull-cluster-api-provider-azure-conformance-custom-builds
pull-cluster-api-provider-azure-e2e	a105ddf	link	true	/test pull-cluster-api-provider-azure-e2e
pull-cluster-api-provider-azure-e2e-aks	a105ddf	link	true	/test pull-cluster-api-provider-azure-e2e-aks

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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