[Snyk] Security upgrade next from 15.5.12 to 15.5.15

[peterj]

Snyk has created this PR to fix 1 vulnerabilities in the npm dependencies of this project.

Snyk changed the following file(s):

• package.json
• package-lock.json

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Allocation of Resources Without Limits or Throttling SNYK-JS-NEXT-15954202	721

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[kristin-kronstain-brown]

Closing: this is now addressed on the kkb-hugo-docs-migration branch (#406, commit 7adc835), which stays on the Next 15 line:

• next → ^15.5.21 (resolves 15.5.22)
• new overrides: postcss → ^8.5.23, sharp → ^0.35.0

All 24 Next.js advisories that applied to 15.5.12 are fixed as of 15.5.21, so the 15.x line is sufficient — npm audit now reports next, postcss, and sharp clean. Verified with npm ci and a full make build (Hugo docs + OpenNext worker); CI is green on #406.

Superseded: #406 lands 15.5.22, which is higher than this PR's target on the same release line.