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

Are there points in the code the reviewer needs to double check?


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 (closed) so it may make sense to add a /templates namespace. Then, move the /license templates endpoint to be underneath, along with .gitignore endpoints.

What are the relevant issue numbers?

#17541 (closed)

Screenshots (if relevant)


Does this MR meet the acceptance criteria?