Add firefox ESR 115 download to /firefox/all/ (Fixes #15083)

[alexgibson]

One-line summary

Adds ESR 115 download button to /firefox/all/

Issue / Bugzilla link

#15083

Testing

http://localhost:8000/en-US/firefox/all/desktop-esr/

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.67%. Comparing base (8ca29f3) to head (0189c06).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
bedrock/firefox/views.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15390      +/-   ##
==========================================
- Coverage   78.68%   78.67%   -0.01%     
==========================================
  Files         156      156              
  Lines        8171     8177       +6     
==========================================
+ Hits         6429     6433       +4     
- Misses       1742     1744       +2     

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[alexgibson]

@robhudson thanks updated