Updated third-party service readme for frontend apps

[rshbhgrg]

refs https://github.com/TryGhost/Team/issues/1719

• unpkg has now been replaced with jsdelivr as the default CDN for loading frontend apps

[codecov]

Codecov Report

Merging #15091 (9a2b4de) into main (5417c9e) will decrease coverage by 1.49%.
The diff coverage is n/a.

❗ Current head 9a2b4de differs from pull request most recent head 488b63f. Consider uploading reports for the commit 488b63f to get more accurate results

@@            Coverage Diff             @@
##             main   #15091      +/-   ##
==========================================
- Coverage   58.19%   56.70%   -1.50%     
==========================================
  Files         732      685      -47     
  Lines       61216    58400    -2816     
  Branches     5309     4917     -392     
==========================================
- Hits        35627    33115    -2512     
+ Misses      25534    25233     -301     
+ Partials       55       52       -3     

Impacted Files	Coverage Δ
...core/server/services/mega/post-email-serializer.js	56.25% <0.00%> (-8.72%)	⬇️
ghost/core/core/server/services/members/service.js	48.06% <0.00%> (-1.09%)	⬇️
ghost/core/core/shared/labs.js	89.28% <0.00%> (-0.10%)	⬇️
...st/core/core/server/services/mega/email-preview.js	0.00% <0.00%> (ø)
...t/core/core/server/services/mega/segment-parser.js	100.00% <0.00%> (ø)
...y-service/lib/api-version-compatibility-service.js
ghost/minifier/index.js
ghost/members-events-service/index.js
...host/mw-session-from-token/lib/SessionFromToken.js
ghost/security/lib/identifier.js
... and 44 more

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