Stop build tests for Debian bookworm

Debian bookworm uses golang 1.19, which is rather outdated. In
preperation of switching to go modules that require a newer go stop
build testing with Debian bookworm. Leaving current Debian stable
(trixie), testing (forky) and arch.

Fixes: #564
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

.github/copilot-instructions.md

@@ -164,7 +164,7 @@ The `.github/workflows/ci.yaml` runs:
    - `go build github.com/sjoerdsimons/ostree-go/pkg/otbuiltin` (pre-build required!)
    - `golangci-lint` with v2.3.1
 
-2. **test** job - Matrix of 4 variants (arch, bookworm, trixie, forky)
+2. **test** job - Matrix of 3 variants (arch, trixie, forky)
    - Build with version: `go build -ldflags="-X main.Version=${DEBOS_VER}" ./cmd/debos`
    - Run unit tests: `go test -v ./...`
    - Verify no skipped tests: `! grep -q SKIP test.out`

.github/workflows/ci.yaml

@@ -52,7 +52,6 @@ jobs:
       matrix:
         variant:
           - arch
-          - bookworm
           - trixie
           - forky
     runs-on: ubuntu-latest