fix(snap): build snap with Go 1.22 to fix CVE-2024-24790 (#448)

Although not shown in the CI (because Trivy is being run with `trivy
fs`, which [doesn't consider the Pebble
binary](https://aquasecurity.github.io/trivy/v0.24.4/vulnerability/detection/language/)),
there currently is a reported CVE in the Pebble snap (CVE-2024-24790).

This CVE seems to have been fixed on newer versions of Go, but we are
still using Go 1.20 to build the Pebble snap.

This PR bumps this build dependency to Go 1.22.

snap/snapcraft.yaml

@@ -33,7 +33,7 @@ parts:
     plugin: go
     source: .
     build-snaps:
-      - go/1.20/stable
+      - go/1.22/stable
     override-build: |
       go generate ./cmd
       CGO_ENABLED=0 go build -trimpath -ldflags=-w -ldflags=-s -o $CRAFT_PART_INSTALL/bin/pebble ./cmd/pebble