Deprecate ALL the octals!

It was possibly an oversight that octal escape syntax was deprecated for plain literals but not for the library's standard interpolator.

It would be nice if at least the f-interpolator warned and suggested alternatives. (This is a follow-on to #8092 and #7292.)

An argument for consistency is that you can add or remove the interpolator from an argless interpolation and not be confused.

It would also be nice if this didn't fail this way:

scala> f"\"
<console>:8: error: exception during macro expansion:
scala.StringContext$InvalidEscapeException: invalid escape character at index 0 in "\"

https://groups.google.com/d/msg/scala-internals/Sn-qUOjEw7g/mDaeQlXR1MEJ