chore(deps): upgrade Go to 1.26 (#5361)

Bump the main module, Dockerfile build stages, and devcontainer to Go
1.26.0. Plugin sub-modules under plugins/ remain on go 1.25 intentionally
(independent modules, untouched in this change).

Also add an explicit actions/setup-go@v6 step (with go-version-file:
go.mod) to the go-lint and go jobs in the CI pipeline. This matches the
golangci-lint-action v4+ requirement that setup-go run before the linter,
and pins the runner Go version to go.mod so CI does not depend on the
ubuntu-latest tools cache picking up Go 1.26.

.devcontainer/devcontainer.json

@@ -4,7 +4,7 @@
 		"dockerfile": "Dockerfile",
 		"args": {
 			// Update the VARIANT arg to pick a version of Go: 1, 1.15, 1.14
-			"VARIANT": "1.25",
+			"VARIANT": "1.26",
 			// Options
 			"INSTALL_NODE": "true",
 			"NODE_VERSION": "v24"