This is purely fixing by meson rebuilds and some regexp fixes. I didn't
touch any semantic.
Basically the "&" XML entity should not have made their way to the
po files. I fixed our script in the previous commit, and now I just
rebuild the po files and clean the entity programmatically so that our
Windows installers for GIMP 2.99.12 release have valid text.

Here is what I did:

(1) I regenerated the pot and po files:

$ meson compile gimp30-windows-installer-pot
$ meson compile gimp30-windows-installer-update-po

(2) I opened all the po files in vim and regexp through them.

Get back the proper Report-Msgid-Bugs-To:

:bufdo! %s$"Report-Msgid-Bugs-To: \\n"$"Report-Msgid-Bugs-To: https://gitlab.gnome.org/GNOME/gimp/issues\\n"$ge|update

No translations were fuzzy in any po files before this change. I remove
all the fuzzy flags, but only if the msgid contains an ampersand, hence
leaving appropriate fuzzy strings in (the ones where the source string
changed since the last translation):

:bufdo! %s/#, fuzzy\n\([^&]*\)&/\1\&/ge|update

Then I replaced & in msgstr (msgid was fixed through the automatic
regeneration thanks to previous commit, but not the translations) with:

:bufdo! %s/&/\&/ge|update

(3) Finally I did a manual `git diff` review to verify all is good.

In particular, I had to fix a bunch of "E&xit". The string is probably
too short so gettext heuristic failed to consider it likely the same as
"E&xit".