FNX-14331 ⁃ Update README.md

[liuche]

We'll be redirecting people from bugzilla, and we've started Fennec rollout, so no time like the present to update references in the readme

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• 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

• 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 #13150 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #13150   +/-   ##
=========================================
  Coverage     28.20%   28.20%           
  Complexity     1034     1034           
=========================================
  Files           417      417           
  Lines         16769    16769           
  Branches       2167     2167           
=========================================
  Hits           4729     4729           
  Misses        11696    11696           
  Partials        344      344           

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[NotWoods]

I think we should continue to say explicitly that Fenix is an internal code name. Otherwise its unclear why the repo is named fenix and not firefox-android.