The Fuchsia Linux/Mac SDK autorollers were paused due to
https://github.com/flutter/flutter/issues/88999 which, as far as we're
aware, has not yet been resolved upstream. Until we hear otherwise,
we've re-paused the rollers and are reverting to the last Fuchsia SDK
confirmed not to have the observatory port issue described in #88999.

This is a revert of both the Linux and Mac SDK rolls:
* Revert "Roll Fuchsia Linux SDK from ZSqn1OAt7... to F0f_c0zno... (#28325)"
  This reverts commit 8a89e93a.
* Revert "Roll Fuchsia Mac SDK from 9xK8HkMEI... to 4p8By4CDl... (#28324)"
  This reverts commit bc0ea465.

Picks up a revert of a change from using python3 to vpython3 for exec
rules in gn. https://github.com/flutter/buildroot/pull/507

Also sets Python runtime to `python3` in `gn.bat` to match the revert to
`python3` in https://github.com/flutter/engine/pull/28313

Issue: https://github.com/flutter/flutter/issues/88719

This reverts commit 93f61889.

This change is similar to:
* https://github.com/flutter/flutter/issues/82728
* https://github.com/flutter/flutter/issues/87073

We are using a more recent Chromium ICU fork commit.

See for reference: https://chromium.googlesource.com/chromium/deps/icu

Flutter needs to use a reasonably recent version of the ICU library. The
Chromium's fork of ICU, which is in use in Flutter as well, has taken in
a few updates since the initial 69.1 release.

Update the DEPS file in github.com/flutter/engine to use the commit ID
`a38aef9142ace942a8bf166020c569f4cda0f8d3` for the ICU library.

This takes in recent patches to the ICU library, most relevant of which
is a fix for Norwegian language support.

Fixes: #flutter/flutter/88600