Move CI to new file system

[nefrathenrici]

Purpose

This PR updates the buildkite pipeline to the new file system (see changes). It also pins julia to 1.11 for the documenter, since it was failing for 1.12.
Original error, caused by this issue :

               ✗ Pkg
  0 dependencies successfully precompiled in 17 seconds. 15 already precompiled.
ERROR: LoadError: ArgumentError: Package OpenSSL_jll [458c3c95-2e84-50aa-8efc-19380b2a3a95] is required but does not seem to be installed:
 - Run `Pkg.instantiate()` to install all recorded dependencies.

---

• I have read and checked the items on the review checklist.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.08%. Comparing base (b102f1f) to head (02767e0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #381      +/-   ##
==========================================
+ Coverage   93.98%   94.08%   +0.10%     
==========================================
  Files          10       10              
  Lines        1630     1624       -6     
==========================================
- Hits         1532     1528       -4     
+ Misses         98       96       -2     

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[odunbar]

LGTM!