Resolve "Move `/licenses` api endpoint to `/templates/licenses`"

## What does this MR do?

It moves the `/licenses`, `/gitignores` and `/gitlab_ci_ymls` API endpoints under the `/templates` namespace

## Why was this MR needed?

In EE we now have somewhat ambiguous API endpoints. `/license` refers to the EE license while `/licenses` (plural) refers to license templates. @DouweM mentioned that we're adding .gitignore templates in #14106 so it may make sense to add a /templates namespace. Then, move the /license templates endpoint to be underneath, along with .gitignore endpoints. 

Closes #17541

See merge request !5717