No issue: Remove milestone from pull request template

[jonalmeida]

We don't use milestones in Fenix, so let's remove it from the PR template.

Pull Request checklist

• [n/a] Tests: This PR includes thorough tests or an explanation of why it does not
• [n/a] Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• [n/a] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

After merge

• [nope] Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[codecov-commenter]

Codecov Report

Merging #14198 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #14198   +/-   ##
=========================================
  Coverage     29.58%   29.59%           
  Complexity     1135     1135           
=========================================
  Files           440      440           
  Lines         17689    17689           
  Branches       2299     2299           
=========================================
+ Hits           5234     5235    +1     
+ Misses        12073    12072    -1     
  Partials        382      382           

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/mozilla/fenix/settings/SupportUtils.kt	76.00% <0.00%> (+2.00%)	13.00% <0.00%> (ø%)

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