Not all of these are necessary to run MW... But they're used in code, so should at *least* be in `suggest` (if not `require` or `require-dev`).
We may want to promote some `suggest` entries too.
Not every one being added will mean admins/dev need to install anything, but some packages do do weird things...
[x] ext-calendar ([[ https://gerrit.wikimedia.org/r/c/mediawiki/core/+/766089 | code change ]] | [[ https://www.mediawiki.org/w/index.php?title=PHP_configuration&diff=5085568&oldid=4683378&diffmode=source | added to documentation ]])
[] ext-curl (in suggests)
[] ext-gd
[] ext-gmp
[x] ext-mysqli (now in suggests)
[] ext-openssl (in suggests)
[] ext-pcntl (now in suggests)
[x] ext-pdo (now in suggests)
[x] ext-pgsql (now in suggests)
[] ext-posix (now in suggests)
[] ext-readline (now in suggests)
[x] ext-simplexml ([[ https://gerrit.wikimedia.org/r/c/mediawiki/core/+/766090 | code change ]])
[] ext-sockets (now in suggests)
[] ext-zlib (now in suggests)