chore: remove deprecated rand.Seed() in testing.docker (#1149)

* chore: remove deprecated `rand.Seed()` in testing.docker

* Use math/rand/v2

* Remove Go 1.21

Go 1.21 doesn't support `math/rand/v2`.

Author: joschi

.github/workflows/ci.yaml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: ["1.21.x", "1.22.x"]
+        go: ["1.22.x"]
     steps:
       - uses: actions/checkout@v4