Update trial guidelines: replace deprecated super form links with support handbook links

Why is this change being made?

The GitLab Support Internal Request "super form" (support-super-form-gitlab-com-support-support-op-651f22e90ce6d7.gitlab.io) has been decommissioned. The Trial Guidelines handbook page still referenced this deprecated URL in two places:

1. "Notes Regarding Trial Licenses" section — linked to the super form for downgrading a trial from Ultimate to Premium.
2. "Extending an Existing Trial" section — linked to the super form for filing internal requests for licensing, subscriptions, trials, and grace period extensions.

Both references are replaced with links to the Support Internal Requests handbook page, which is the single source of truth for the current L&R Internal Request workflow (including Global vs. US Gov links and form selection instructions). Duplicated form instructions are removed from the trial guidelines page.

Closes #671 (closed)

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct
  • The when to get approval handbook section explains when DRI approval is required
  • The who can approve handbook section explains how to identify the DRI
  • If the MR does not require DRI approval, consider asking someone on your team, such as your manager.
  • The approver may merge the MR. If they approve but don't merge, you can merge.
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Division: If the update affects your division, share the MR in your division Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

Commits

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