user group condition selectable when applying pc

[i-just]

Description

Allow the User Group condition to be selectable if we’re applying external project config changes (if the changes are likely to add user group(s).

At the moment, if you’re installing a new project using the existing config (or if you were to add the first user group and a rule that depends on it to an existing project in one go), the User Group condition won’t be added as the craft\elements\conditions\users\GroupConditionRule->isSelectable() will always be false (because there are no groups for the project yet). Since adding user groups can’t happen earlier on, we need to check if the user groups are likely to be added when applying changes, and if so, allow this rule to be selectable.

Related issues

#15037