Renames the old analysis_options.yaml to analysis_options_legacy.yaml,
replacing it with a slightly modified copy of flutter/flutter's analysis
options.

Each plugins has a temporary local analysis_options.yaml that points to
the legacy version. This allows for inceremental conversion on a
per-plugin basis, which should make the problem more tractable.

Since this hasn't yet been enabled for any packages, it's likely that as
it is we'll find a few local modification we need to make to the root
analysis_options (e.g., things that conflict with 'dart format').

Part of flutter/flutter#76229