chore(dev): upgrade dev container to v0.21

See [release notes](https://github.com/qdm12/godevcontainer/releases/tag/v0.21.0)

Notably:
- Go upgraded from 1.23 to 1.25
- golangci-lint upgraded to v2.4.0
- Alpine upgraded from 3.20 to 3.22
- Disable package comment requirement by gopls' staticcheck
- Pull container image from ghcr.io

.devcontainer/devcontainer.json

@@ -82,6 +82,9 @@
                 "gopls": {
                     "usePlaceholders": false,
                     "staticcheck": true,
+                    "ui.diagnostic.analyses": {
+                        "ST1000": false
+                    },
                     "formatting.gofumpt": true,
                 },
                 "go.lintTool": "golangci-lint",