Bump Jenkins version for UI tests to 2.303.3

[uhafner]

Latest LTS.

[codecov]

Codecov Report

Merging #373 (fc63527) into master (c68be1d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #373   +/-   ##
=========================================
  Coverage     80.12%   80.12%           
  Complexity      157      157           
=========================================
  Files            22       22           
  Lines           644      644           
  Branches         69       69           
=========================================
  Hits            516      516           
  Misses          100      100           
  Partials         28       28           

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