Attempt to improve merge-conflicts modal

What does this MR do?

This MR removes merge conflicts resolving example from Checkout modal window in merge requests. These instructions are incorrect and shouldn't be followed. The link to the docs in the window already has proper instructions in place.

It also fixes minor typos regarding this modal window and removes any mentions of merging into target branch to resolve conflicts.

Additional details

This MR is really more frontend than documentation, so I'll do my best to kickstart it, then seek help to move it forward. I lack technical knowledge on two fronts: 1) doing the FE work to change the modal(s?) and 2) deep enough Git knowledge to be certain I'm recommending the right instructions to ALL users who see this merge-conflict modal window.

Notes for FE reviewer / helper:

• I haven't been able to view my changes locally. GDK is stuck at Checking if merge request can be merged… and at some point I have to acknowledge I'm trying to do non-docs things here
• #383823 (closed) contains an excellent description of the current state. Please read it.
• Now compare the instructions in the modal to the instructions in the docs, because they're different:
  • the modal describes checking out master and using git merge --no-ff to merge the feature branch into it. Not all users have merge rights on the default branch (master/main), so these instructions won't work for everyone.
  • the docs for 'resolve conflicts from the command line' instruct the user to instead rebase, then use git push --force-with-lease to their feature branch. We could then encourage the user to return to the GitLab UI to complete the merge request.
• My impression from @ipedowitz is that we should update the instructions in app/assets/javascripts/vue_merge_request_widget/components/mr_widget_how_to_merge_modal.vue to use the procedure we have in the docs, but updating mr_widget_how_to_merge_modal.vue is out of scope for me as a technical writer, so I need to hand it off.
• Please, please double-check the instructions as part of the review process. This modal matters to a lot of users, and we need to get the instructions right. As a writer, I can help you polish the language and presentation, but I'm dependent on the engineers in my groups to make sure the info I'm polishing is actually correct.

Screenshots

Before	After

Related issues

• Related to Add link to docs about conflicts in the resolve... (#344883 - closed), and might even close it depending on what's done here
• Related to !73565 (comment 722054762)
• Related to Improve displayed instructions for local merge ... (#347154 - closed)
• Related to Merge conflict instructions don't work to fix l... (#383823 - closed)

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