[BLOCKED] Add support for mijn.host DNS plugin

[Sausageroll2077]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Adds mijn.host as a DNS validation provider, resolving #606.

Changes:

• Add certbot-dns-mijn-host to Dockerfile and Dockerfile.aarch64 (kept in alphabetical order).
• Add a credentials template at root/defaults/dns-conf/mijn-host.ini.
• Add mijn-host to the DNSPLUGIN options list and a changelog entry in readme-vars.yml.

The plugin follows certbot's standard naming convention (authenticator dns-mijn-host, dns-mijn-host-credentials, dns-mijn-host-propagation-seconds), so no changes to the certbot init logic were required.

Benefits of this PR and context:

Closes #606. Adds DNS-01 validation support for another provider (mijn.host), requested by users.

⚠️ Blocked: This is currently a draft because the released certbot-dns-mijn-host (0.0.8) on PyPI pins josepy==1.14.0, which conflicts with the certbot version SWAG ships (josepy>=2.0.0) and breaks the image build with ResolutionImpossible. The fix is already open upstream as mijnhost/certbot-dns-mijn-host#15 (removes the pin). This PR should not be merged until that PR is merged and a new version is published to PyPI.

How Has This Been Tested?

Built the full image locally against the patched plugin (the branch from mijnhost/certbot-dns-mijn-host#15, which removes the josepy pin):

• pip resolves the entire plugin set cleanly — certbot 5.6.0, acme 5.6.0, josepy 2.2.0, certbot-dns-mijn-host 0.0.8.
• The image builds and exports successfully.
• certbot plugins --authenticators correctly registers dns-mijn-host:

  * dns-mijn-host
  Description: This plugin configures a DNS TXT record to respond to a dns-01
  challenge using the mijn.host API
  Interfaces: Authenticator, Plugin
  Entry point: EntryPoint(name='dns-mijn-host',
  value='certbot_dns_mijn_host.mijn_host:Authenticator', group='certbot.plugins')
  

Actual certificate issuance against a live mijn.host domain has not been tested, as I don't have a mijn.host account — the issue reporter (#606) may be able to validate end-to-end.

Source / References:

• Closes [FEAT] DNSPLUGIN for mijn.host #606
• Plugin: https://github.com/mijnhost/certbot-dns-mijn-host (certbot-dns-mijn-host on PyPI)
• Upstream fix this depends on: removal of josephy requirement mijnhost/certbot-dns-mijn-host#15