test(e2e): Add NextJS 15 E2E test app

[Lms24]

This PR adds a new NextJS 15 e2e test app. The reason is that we want to test against TypeScript-based next.config.ts files (to avoid bugs like #798). This is a NextJS>=15 feature though, so our existing Next 14 can't be adapted. Therefore this PR also supersedes #810.

This PR also renames the existing "NextJS" e2e test app to "NextJS-14", so that we can distinguish the two apps properly in the yarn test:e2e command as well as visually on first glance

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.15%. Comparing base (4f49ad8) to head (885082b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #852    +/-   ##
========================================
  Coverage   51.15%   51.15%            
========================================
  Files          53       53            
  Lines        3519     3519            
  Branches      816      827    +11     
========================================
  Hits         1800     1800            
+ Misses       1718     1592   -126     
- Partials        1      127   +126     

Flag	Coverage Δ
e2e-tests	80.85% <ø> (ø)
unit-tests	49.91% <ø> (ø)

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