text: Title: Deprecate setting tags starting with a hyphen like `-tag` in `Test Tags`
  Body: The plan is to allow using the `-tag` syntax in `Test Tags` like
  
  ```robotframework
  *** Settings ***
  Test Tags        -example
  ```
  for removing tags set earlier, for example, in suite initialization files or by using the `--set-tag` option. That functionality is planned for RF 8.0 (#5250).
  
  This issue covers deprecating using tags like `-example` as literal tag names. Such usage should cause a deprecation warning to be emitted with a note that escaping like `\-example` is possible.