Chore: Rollback to jest@26.x

[gu-stav]

What does it do?

Downgrade jest to jest@26.x, because we experience too many issues, such as #13843

Why is it needed?

To have a stable release with all tests passing.

[codecov]

Codecov Report

Merging #13861 (c197517) into master (cd963b7) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head c197517 differs from pull request most recent head c77a54f. Consider uploading reports for the commit c77a54f to get more accurate results

@@            Coverage Diff             @@
##           master   #13861      +/-   ##
==========================================
+ Coverage   56.66%   56.69%   +0.03%     
==========================================
  Files        1178      974     -204     
  Lines       28988    21660    -7328     
  Branches     5418     3504    -1914     
==========================================
- Hits        16425    12281    -4144     
+ Misses      10799     8390    -2409     
+ Partials     1764      989     -775     

Flag	Coverage Δ
front	56.69% <ø> (-3.17%)	⬇️
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/core/admin/admin/src/app.js	0.00% <0.00%> (ø)
packages/core/admin/admin/src/index.js	0.00% <0.00%> (ø)
packages/core/email/admin/src/index.js	0.00% <0.00%> (ø)
packages/core/upload/admin/src/index.js	0.00% <0.00%> (ø)
packages/plugins/i18n/admin/src/index.js	0.00% <0.00%> (ø)
packages/plugins/i18n/admin/src/schemas.js	0.00% <0.00%> (ø)
packages/plugins/i18n/admin/src/permissions.js	0.00% <0.00%> (ø)
packages/core/upload/admin/src/pages/App/index.js	0.00% <0.00%> (ø)
...ckages/plugins/i18n/admin/src/middlewares/index.js	0.00% <0.00%> (ø)
...kages/core/content-type-builder/admin/src/index.js	0.00% <0.00%> (ø)
... and 409 more

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[Convly]

LGTM