Replace mimemagic dependency and introduce a Gitlab::Utils::MimeType class

What does this MR do?

Replaces our direct usage of mimemagic with the gem ruby-magic. However it uses libmagic under the covers, which can vary per system. So we need to include our own version which pulls the source and compiles it. See ruby-magic!1 (merged)

Note: this MR is built on top of !57421 (merged), which adds the base support for Gitlab::Utils::MimeType. That MR needs to be merged first.

Related #325851 (closed)

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