Deprecate StringSubject.doesNotMatch?

As noted in https://github.com/google/truth/pull/789#issuecomment-732213007, it's _very_ frequently misused.

Users who want that behavior would likely be better served by having to opt into it more explicitly with a call like `doesNotContainMatch("(?s)^.*something.*$")`. Unfortunately, even that is likely to lead to misuse: If users omit `(?s)`, then `.` does not match newlines, so any string with a newline will pass a test like `doesNotContainMatch("^.*something.*$")` :( Perhaps our regex methods would ideally have enabled `(?s)` by default. But then that would be different from the `Pattern`-accepting overloads. There is no great solution here.