Rename authentication generator templates, add missing .tt extensions

Generator templates are typically named using a `.tt` extension to help
differentiate them from actual code. Without the extension, editors and
other tools can accidentally consider these files as code from the
gem, and it can affect their functionality. For example, it affected
editors when trying to navigate to models named "User", as they would
now offer the generator template as a possible target.

[Fix rails#53106]

Author: hjwylde