[release-1.20] Fix mishandling of azure:// prefix on AzureMachine UserAssignedIdentities

[nojnhuh]

What type of PR is this?
/kind bug

What this PR does / why we need it:

Manual cherry-pick of #5930:

This PR fixes a regression caused by #5780 which didn't account for the possibility that user assigned identity IDs in CAPZ templates can be prefixed with azure://.

This PR also updates all of our reference and test templates to use the azure:// prefix since it's documented that way in the API: https://capz.sigs.k8s.io/reference/v1beta1-api#UserAssignedIdentity

With the same caveat as #5932:

This PR doesn't make the template changes as widely as the original PR. I reverted all of the changes to the templates/flavors directory to avoid any unnecessary churn there, and those changes matriculated to some of the test templates. I think there's about a 50/50 split in the test templates that use the azure:// prefix in user-assigned identity IDs and those that don't. I could try to make more of the test templates use the prefix but that effort doesn't seem worthwhile.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Ref #5929

Special notes for your reviewer:

TODOs:

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

Release note:

Fixed a regression where user-assigned identities with IDs prefixed with `azure://` for AzureMachines were misidentified as missing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.81%. Comparing base (b8f62ac) to head (5e557bf).
⚠️ Report is 6 commits behind head on release-1.20.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-1.20    #5937   +/-   ##
=============================================
  Coverage         52.80%   52.81%           
=============================================
  Files               279      279           
  Lines             29602    29602           
=============================================
+ Hits              15632    15634    +2     
+ Misses            13156    13154    -2     
  Partials            814      814           

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[nojnhuh]

Bootstrap flake

/retest

[mboersma]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 4ae8356f9ad13bc8ef96e7836e5722ffe9a6e4a7

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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