Clarify Cert Manager requirement

What does this MR do?

This phrase is somewhat confusing:

Cert Manager is a prerequisite for the GitLab Operator, while the GitLab Helm chart bundles it.

Initially, tt was not clear to me that we can't use the cert-manager that is bundled with the chart. I actually thought that we could.

So this MR tries to be more explicit about stating that:

1. cert-manager is required.
2. the one that comes with the chart is not an option, because cert-manager needs to be present in the cluster previous to the chart installation.

Additionally, I'm also considering two other things:

• Override Cert Manager mentions to cert-manager, as this seems to be the chosen product name upstream: https://cert-manager.io/
• Remove the NOTE: annotation for the cert-manager clarification and make it a regular paragraph, as the note is going to become much bigger and we generally avoid the overuse of NOTE:.

All this, to be consulted with a technical-writter.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

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