[codex] bump mautic to 7

[henmohr]

What changed

• Bumped the compose/runtime defaults from Mautic 5 to Mautic 7.
• Updated the entrypoint fallback used to bootstrap new installs.
• Updated README and release workflow examples to reference Mautic 7.

Validation

• Built librecodecoop/mautic:7.1.1-apache locally.
• Started the stack with docker compose up -d.
• Populated the mounted Mautic volume with a Mautic 7.1.2 installation for a real runtime check.
• Verified http://localhost/login returns 302 Found and redirects to /index.php/installer.

Notes

• The local volumes/mautic contents used for testing are ignored by git and were not committed.