feat: allow configuration of mpm prefork in apache + mod_php #212

Job	Run time
hadolint	6s
Bats Testing	4s
pixelfed (8.3, dev, bookworm, apache, apache)	3m 28s
pixelfed (8.3, staging, bookworm, apache, apache)	3m 40s
pixelfed (8.3, dev, bookworm, fpm, nginx)	1m 50s
pixelfed (8.3, staging, bookworm, fpm, nginx)	2m 42s
pixelfed (8.4, dev, bookworm, apache, apache)	3m 30s
pixelfed (8.4, dev, bookworm, fpm, nginx)	3m 4s
pixelfed (8.4, staging, bookworm, apache, apache)	4m 16s
pixelfed (8.4, staging, bookworm, fpm, nginx)	1m 43s
	24m 23s

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