text: Title: Deprecate `compile_uri_template()`
  Body: The `compile_uri_template()` function is not consistent with compiled router regarding tailing slash.
  
  > I don't know either what the plan for `compile_uri_template()` is. I think it is left as a utility method for building custom routers, and I suppose, may continue to exist as such.
  
  > This PR does a good job at explaining the _status quo_, so maybe let's just merge it, and create a new issue for making this method slightly more consistent with `CompiledRouter`.
  
  _Originally posted by @vytas7 in https://github.com/falconry/falcon/pull/1961#pullrequestreview-777018325_
  
  The options may be
  - to leave it as is, with the warning provided by the above referenced pr
  - add a flag to select what behaviour to have
  - deprecate and eventually remove it, since it's not used internally by falcon.