docs: reference new repo for OpenShift installation

[fgiloux]

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cilium/cilium-olm has been deprecated and archived in favour of isovalent/olm-for-cilium. This PR updates the documentation page for OpenShift installation to reflect it. It also adds a few common pitfalls.

[joestringer]

Sorry, I didn't realize you also had a proposal out in this area and that it conflicted with #29300 (fixes #24270). Perhaps you and @learnitall (and others interested) could get together and figure out the right path forward?

[learnitall]

@fgiloux we essentially moved the guide over to https://github.com/isovalent/olm-for-cilium. The note that you added in this PR would be a nice addition there if you'd like to open another PR.

.. note::

   OpenShift installations use a different default port than Cilium for VXLAN: UDP 4789 (IANA) instead of UDP 8472 (Linux).
   Low range ports are also often blocked on OpenShift clusters. The port for cluster health needs to be open or
   configured to a higher range, e.g. ``clusterHealthPort: 9940``.
   The configuration used for conformance tests in isovalent/olm-for-cilium can be leveraged for a quick setup.

[fgiloux]

@learnitall I was aware of the doc move PR. It was open for 2.5 months and merged after I opened this one. I will open a PR in olm-for-cilium.