fix: Use correct case for PO file header MIME-Version

[datafoo]

Fix #1071

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/lingui-js/js-lingui/nt5JdykbPg9Yvy9N1F1gqic8StyF
✅ Preview: https://js-lingui-git-fork-datafoo-fix-po-mime-header-case-lingui-js.vercel.app

[codecov]

Codecov Report

Merging #1074 (d902264) into main (9990eba) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1074   +/-   ##
=======================================
  Coverage   83.20%   83.20%           
=======================================
  Files          53       53           
  Lines        1620     1620           
  Branches      441      441           
=======================================
  Hits         1348     1348           
  Misses        159      159           
  Partials      113      113           

Impacted Files	Coverage Δ
packages/cli/src/api/formats/po-gettext.ts	85.83% <ø> (ø)
packages/cli/src/api/formats/po.ts	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9990eba...d902264. Read the comment docs.