fix: prefixLength minimum value in avm/res/network/public-ip-prefix

[krbar]

Description

Updates the minimum value of the prefixLength parameter to /21 as per:

• https://learn.microsoft.com/en-us/azure/virtual-network/ip-services/public-ip-address-prefix

If you are deriving a Public IP Prefix from a Custom IP Prefix (BYOIP range), the prefix size can be as large as the Custom IP Prefix.

• https://learn.microsoft.com/en-us/azure/virtual-network/ip-services/custom-ip-address-prefix :

A custom IPv4 Prefix must be between /21 and /24

Fixes #3367

Pipeline Reference

Pipeline

Type of Change

• Update to CI Environment or utilities (Non-module affecting changes)
• Azure Verified Module updates:
  • Bugfix containing backwards-compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in version.json:
    • Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
    • The bug was found by the module author, and no one has opened an issue to report it yet.
  • Feature update backwards compatible feature updates, and I have bumped the MINOR version in version.json.
  • Breaking changes and I have bumped the MAJOR version in version.json.
  • Update to documentation

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• I have run Set-AVMModule locally to generate the supporting module files.
• My corresponding pipelines / checks run clean and green without any errors or warnings

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