We'd like to prepare the ground work for removing Skia GPU from iOS applications.

To do that, we'll need to stop support --no-enable-impeller (tracked elsewhere, this is google3 only).

• Flip the default (to match the 3P tool), i.e. explicitly required (the equivalent of) --no-enable-impeller.
• Announce the changes.
• Add and enforce an allow-list.
• Move folks over (i.e. remove opt-outs).
• Remove the ability entirely to use --no-enable-impeller.

I am starting on only the first part (flip the default), and the rest is subject to change (we're discussing it internally).

Note: This issue is locked as closed to flutter-hackers only.

/cc @chinmaygarde @dnfield @jonahwilliams @xster @CaseyHillers