Add userdir-ldap to automation

[samuelallan72]

This makes some changes to the templates that will affect other charms too:

• unpin tox-version for lint; all our lints should work with tox 4 now
• remove step to install latest tox version; the actions-operator action does this for us
• move model-constraints from matrix; we can dynamically derive this value from the environment
• checkout with submodules for charm relesae - some charms have submodules