[ruby/erb] Revert "Remove safe_level and further positional arguments (https://github.com/ruby/erb/pull/7)"

This reverts commit https://github.com/ruby/erb/commit/5133efa06f0603ae79292f3b2b942957bc8a442e.

While we already handled this deprecation in many libraries, we noticed
that some (e.g. sprockets) relied on the format of `ERB.version` and
https://github.com/ruby/erb/commit/2b4182eb108b9e42fa30bcfa41931896132f88b8 broke such handling.

Given that the `ERB.version` change was released at 3.1 and it's
obviously new, I'll skip this removal in 3.2 and postpone this to a
future version.