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.

It is now merged but there is no release on Pypi yet.

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