[WIP] Default azure-cni-v1, spot, and custom-vnet e2e templates to Azure Linux 4

[mboersma]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This begins making Azure Linux 4 the default operating system for CAPZ e2e tests by flipping the control plane and Linux worker nodes of three CI cluster-template flavors to the capi-azurelinux-4-gen2 community gallery image:

• prow-azure-cni-v1 (REQUIRED)
• prow-spot (OPTIONAL)
• prow-custom-vnet (OPTIONAL)

Inspired by #6312, but rather than adding a single optional Azure Linux 4 spec, we exercise Azure Linux 4 across multiple existing specs. The AZL4 deltas (compute gallery image, marketplace removal, ca-certificates/iptables bootstrap, and cloud-provider-azure caCertDir) are factored into reusable patches under templates/test/ci/patches/azl4-*.yaml, modeled on the existing Azure Linux 3 overlay.

Scope is intentionally limited to flavors that have no ci-version/azl3 child overlays, so the change is self-contained and low-risk. The basic "Creating a highly available cluster [REQUIRED]" spec and the optional Azure Linux 3 spec are left on their existing images, end-user-facing templates remain Ubuntu-based, and Windows worker nodes are unaffected.

Follow-up PRs can extend Azure Linux 4 to the machine-pool, ci-version, and conformance flavors, which require a leaf-overlay approach to avoid disrupting their child templates.

Which issue(s) this PR fixes:

Special notes for your reviewer:

AZL4_VERSION is set in the e2e BeforeEach (keyed to the Kubernetes version, matching the existing AZL3_VERSION convention). These templates depend on the capi-azurelinux-4-gen2 image being published in the community gallery.

TODOs:

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

Release note:

Default azure-cni-v1, spot, and custom-vnet e2e templates to Azure Linux 4

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.85%. Comparing base (f5bc974) to head (b4f3e8d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6330   +/-   ##
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  Coverage   43.85%   43.85%           
=======================================
  Files         291      291           
  Lines       25344    25344           
=======================================
  Hits        11114    11114           
  Misses      13457    13457           
  Partials      773      773           

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[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-dual-stack-with-ci-artifacts	b4f3e8d	link	false	/test pull-cluster-api-provider-azure-conformance-dual-stack-with-ci-artifacts
pull-cluster-api-provider-azure-e2e	b4f3e8d	link	true	/test pull-cluster-api-provider-azure-e2e

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