fix: Encoding Firebase push notification keys

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Approach

• Firebase encoding keys should use underscores instead of dashes
• Bump to latest Xcode
• Only build linux for latest 5.x as 6.x forces Swift 6 conversion

TODOs before merging

• Add tests
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.82%. Comparing base (4799684) to head (1bd7322).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   91.65%   91.82%   +0.16%     
==========================================
  Files         179      179              
  Lines       15874    15919      +45     
==========================================
+ Hits        14550    14617      +67     
+ Misses       1324     1302      -22     

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