[jest-runner] Don't print warning to stdout when using --json

[cpojer]

Summary

This warning should go to stderr, otherwise when you have a test that leaks while using --json it will produce invalid JSON when reading the stdout of a Jest process.

Test plan

This regressed in Jest through #8206 and made Jest on CI at FB fail. Verified this change fixes it.

[codecov-io]

Codecov Report

Merging #9843 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #9843      +/-   ##
==========================================
- Coverage   64.52%   64.51%   -0.02%     
==========================================
  Files         290      290              
  Lines       12325    12325              
  Branches     3048     3048              
==========================================
- Hits         7953     7951       -2     
- Misses       3732     3733       +1     
- Partials      640      641       +1     

Impacted Files	Coverage Δ
packages/jest-runner/src/index.ts	64.00% <0.00%> (ø)
packages/expect/src/utils.ts	94.96% <0.00%> (-1.26%)	⬇️

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[github-actions]

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