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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