[FIXES JENKINS-17281] Adding configuration options for the filters used to search for groups.

- It is somewhat confusing that there are two `group search filters` so I have decided to rename one.
- The new name for the `groupSearchFilter` that is controlled from `LDAPBindSecurityRealm.groovy` is the
  `groupMembershipFilter` as this filter is used to determine what groups a specific user is a member of
- That leaves `groupSearchFilter` as a nice clean name for the filter to search for named groups.
- This should still respect any existing configuration, i.e. leaving these fields blank will leave the
  existing defaults or existing overrides in place... but it will make life easier for users going forward
- Took quite some digging to figure out exactly what these filters were for... hopefully I have left things
  in a more obvious framing for anyone else following
- I would like a better way to apply the `groupMembershipFilter` override, but this was the cleanest way
  I could maintain backwards compatibility