fix: bump qs minimum to 6.15.2

[cyphercodes]

Bumps the qs dependency minimum from ^6.14.2 to ^6.15.2 so Express resolves the patched version for GHSA-q8mj-m7cp-5q26.

Fixes #7304

Verification:

• npm install --package-lock=false
• npm ls qs
• npm run lint
• npx mocha --require test/support/env --reporter spec --check-leaks test/req.query.js test/express.urlencoded.js
• npm test
• npm audit --omit=dev
• git diff --check