Add Oracle Linux installation instructions (#17436)

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(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

I forgot the guide applies to Oracle Linux as well. In fact, I ran a
small homeserver on OEL a few months back.

I did minimal installations on Rocky and OEL on VirtualBox and noticed
Codeready/Powertools repos aren't required, so I removed those commands
from the guide. I switched `RHEL`-references to `EL`.

#17423 was merged before I remembered about OEL but a new PR shouldn't
hurt :)

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

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