FindGTest: Deprecate result variables and update documentation

This updates the module documentation and deprecates some result
variables in favor of using imported targets.

Changes:
- Synced module documentation with other similar find modules.
- Reworded and updated descriptions.
- Renamed "Cache Variables" to "Hints".
- Added info about config mode.
- Added separate section "Deprecated Items".
- Mention of Threads::Threads imported target removed.