[RHELC-1780] Fix bad grub2-mkconfig call on UEFI EL9+

[danmyway]

The output of the grub2-mkconfig call suggests to not use /boot/efi/EFI/redhat/grub.cfg as an output file, but use the /boot/grub2/grub.cfg instead.
Check what major version of OS is running and assign the grub config variable based on that.
Additionally, make the grub config into a variable inside the remediation - do not always advise to use the /boot/efi/EFI/redhat/grub.cfg
Reformat the action message.

Jira Issues:

• RHELC-1780

Checklist

• PR has been tested manually in a VM (either author or reviewer)
• Jira issue has been made public if possible
• [RHELC-] or [HMS-] is part of the PR title
• Label depicting the kind of PR it is
• PR title explains the change from the user's point of view
• Code and tests are documented properly
• The commits are squashed to as few commits as possible (without losing data)
• When merged: Jira issue has been updated to Release Pending if relevant

[has-bot]

This PR does not require integration tests to be run.

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Comment generated by an automation.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.12%. Comparing base (02fa48a) to head (6a69cd2).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1444   +/-   ##
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  Coverage   96.12%   96.12%           
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  Files          72       72           
  Lines        5186     5186           
  Branches      896      896           
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  Hits         4985     4985           
  Misses        119      119           
  Partials       82       82           

Flag	Coverage Δ
centos-linux-7	91.65% <100.00%> (ø)
centos-linux-8	92.50% <100.00%> (ø)
centos-linux-9	92.62% <100.00%> (ø)

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[danmyway]

For reviewers:
The grub2 regeneration action message formatting was kind of awkward.
See:

(WARNING) UPDATE_GRUB::GRUB2_CONFIG_CREATION_FAILED - The grub2-mkconfig call
    failed to complete
     Description: The grub2-mkconfig call failed with output: 'Running
`grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg' will overwrite the GRUB
wrapper.
    Please run `grub2-mkconfig -o /boot/grub2/grub.cfg' instead to update
grub.cfg.
    GRUB configuration file was not updated.
    '. The conversion will continue but there may be issues with the current
grub2 config and image formats.
     Diagnosis: N/A
     Remediations: If there are issues with the current grub2 config and image
we recommend manually re-generating them with 'grub2-mkconfig -o
/boot/efi/EFI/redhat/grub.cfg' and'grub2-install [block device, e.g. /dev/sda]'.

[danmyway]

The plan which triggered the issue is scheduled here: https://artifacts.osci.redhat.com/testing-farm/e931c267-6758-43a4-bb4b-a888885b51e0

[danmyway]

EL9 plan is PASSing
Triggered the same test plan on EL8 just for a good measure
https://artifacts.osci.redhat.com/testing-farm/eadc1b04-f550-4f2e-9885-f1913653ea68/