Use always the newest GraphQL playground version

[salwator]

I want to merge this change because...

Current version of playground in saleor is hardcoded to 1.7.8. We should use newer (1.8) instead, preferably refer to newest version on CDN.

One important feature available in newer version is browsing schema through dedicated playground menu.

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #3971 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3971   +/-   ##
=======================================
  Coverage   91.29%   91.29%           
=======================================
  Files         267      267           
  Lines       14508    14508           
  Branches     1395     1395           
=======================================
  Hits        13245    13245           
  Misses        894      894           
  Partials      369      369

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[maarcingebala]

@salwator Thanks for opening this PR! Can you please update the changelog file as well?