text: Title: Deprecate using arguments starting with `$`, `@`, `&` and `%` as literal values
  Body: We are planning to support `$var` syntax in addition to `${var}` in the future (#4674). The plan was to do that already in RF 7.0, but it turned out that this would be too badly backwards incompatible. Probably the biggest problem would be that the Browser library accepts passwords in format `$secret` and resolves the variable value internally to avoid the value being logged automatically. After the planned change `$secret`, would be considered exactly the same as `${secret}` meaning that a confidential value could leak.
  
  We still believe that supporting the `$var` syntax is a good idea, but we need to first deprecate using values starting with `$` or any other variable identifier. That means that using, for example,
  ```
  Log Many    $value    @another
  ```
  will cause a deprecation warning and the value needs to be escaped like
  ```
  Log Many    \$value    \@another
  ```
  