Clarify firewall requirements for Jira Cloud app

What does this MR do?

This MR clarifies which type of firewall rules (inbound/outbound) need to be applied to which IP ranges. The MR also makes the reverse proxy firewall requirements more streamlined with the general firewall requirements.

At the moment, the Atlassian Marketplace app firewall requirements in the docs are incorrect as Jira Cloud won't actually contact the self-managed instance directly.

See https://gitlab.slack.com/archives/C04RDL3MEH5/p1727327844822649?thread_ts=1727217164.408439&cid=C04RDL3MEH5::

[at]anton: [at]lduncalfe can you fact check me there, does that sound correct? Is there a situation where Atlassian Jira would contact the self-managed instance directly if you have installed the Jira Cloud app via the Atlassian Marketplace?

[at]lduncalfe: [at]anton Yes that’s correct, there wouldn’t be a situation where Jira would contact the GitLab instance directly if the instance had installed the GitLab for Jira Cloud app via the Atlassian Marketplace. The contact from Jira is limited to the app lifecycle events, and there’s a redirect for branch creation to the instance. When installed from the Marketplace these are both handled by GitLab.com. (edited)

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