test: remove TS 6 RC from test matrix

[kirkwaiblinger]

PR Checklist

• Addresses an existing open issue: fixes #000
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Since TS 6 is "latest" now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.82%. Comparing base (e0a8340) to head (f0ac2d4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #907   +/-   ##
=======================================
  Coverage   58.82%   58.82%           
=======================================
  Files          29       29           
  Lines         901      901           
  Branches      283      283           
=======================================
  Hits          530      530           
  Misses        287      287           
  Partials       84       84           

Flag	Coverage Δ
5.5.4	58.71% <ø> (ø)
5.9.3	58.71% <ø> (?)
latest	58.82% <ø> (ø)
rc	?

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[github-actions]

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Issue Reference

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