fix: graphql@16 has peer dep conflicts with [email protected]

[trentm]

This peer dep conflict breaks our dist build (make -C .ci dist).
Fix the conflict by backing up to graphql@15 for now -- until we
separate out testing deps to separate package dirs, or until
express-graphql updates to support graphql@16.
We still test graphql@16 with our TAV tests.

Fixes: #2773

[apmmachine]

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• Start Time: 2022-06-14T16:14:24.955+0000

• Duration: 22 min 5 sec

Test stats 🧪

Test	Results
Failed	0
Passed	258073
Skipped	0
Total	258073

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