Update karma dependency for JS tests

[TimKam]

As discussed here.
The version of the Karma test runner we use for JS unit tests has dependencies with low-moderate security vulnerabilities. This is not a security issue for Sphinx, because Sphinx does not include these dependencies in any release. But anyway, as Karma has now updated the corresponding dependencies, we should update our Karma version.

Relates

• 5186 set up JavaScript test infrastructure #5207 (comment)
• Karma Dependencies Security Vulnerabilities (NPM Audit) karma-runner/karma#2994

[codecov]

Codecov Report

Merging #5295 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5295   +/-   ##
=======================================
  Coverage   82.21%   82.21%           
=======================================
  Files         296      296           
  Lines       39346    39346           
  Branches     6058     6058           
=======================================
  Hits        32349    32349           
  Misses       5669     5669           
  Partials     1328     1328

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[tk0miya]

LGTM!