test(e2e): Bump `nextjs-t3` to next 15

[github-actions]

Note

The pull request "test(e2e): Bump nextjs-t3 to next 15" was created by @chargome but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Latest t3 runs on next 15 so this makes more sense imo.

[linear]

JS-1621

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #19130, which was included in the 10.39.0 release.