Update ESR link on /welcome/20/ page to link to /firefox/all/desktop-esr/ (Fixes #15501)

[alexgibson]

One-line summary

The folks working on the root-ca-succession project have agreed that this feels like the more appropriate link for ESR users.

Issue / Bugzilla link

15501

Testing

• http://localhost:8000/en-US/firefox/welcome/19/
• http://localhost:8000/en-US/firefox/welcome/20/
• http://localhost:8000/en-US/firefox/welcome/21/
• http://localhost:8000/en-US/firefox/welcome/22/
• http://localhost:8000/en-US/firefox/welcome/23/
• http://localhost:8000/en-US/firefox/welcome/24/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.71%. Comparing base (e4df84e) to head (e0bf704). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #15502 +/- ## ======================================= Coverage 78.71% 78.71% ======================================= Files 156 156 Lines 8197 8197 ======================================= Hits 6452 6452 Misses 1745 1745 ```

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