`contributing.md` links to "good first issue" GitHub issues label, which is empty

[strugee]

Vision statement / What / Requirements

Basically what the title says. See https://github.com/mozilla-lockwise/lockwise-android/blob/master/docs/contributing.md#how-to-contribute-code.

Impact

Potential contributors may be confused while attempting to contribute and bail out instead of sending patches.

Acceptance criteria

A few bugs are labeled "good first issue", or this section is removed from contributing.md to avoid confusing potential contributors.