CI: Run conformance tests only once

[snazy]

It's just not necessary to run the conformance tests against multiple
different Java versions, because those tests do not hit any code path
in CEL-Java that any way dependends on the Java version.

Running the conformance tests only for one Matrix job also improves
CI runtime, because Bazel's cache depends on the Java version.

[codecov]

Codecov Report

Merging #202 (e52822b) into main (c44565d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #202   +/-   ##
=========================================
  Coverage     80.37%   80.37%           
  Complexity     1835     1835           
=========================================
  Files           100      100           
  Lines          7899     7899           
  Branches       1143     1143           
=========================================
  Hits           6349     6349           
  Misses         1033     1033           
  Partials        517      517           

Flag	Coverage Δ
java	80.37% <ø> (ø)

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[XN137]

yaml changes lgtm but i cant judge whether conformance tests could show different behavior depending on the java version or not, so have to trust you on this one ;)