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.

.github/workflows/pipeline.yml

@@ -64,6 +64,10 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
+      - uses: actions/setup-go@v6
+        with:
+          go-version-file: go.mod
+
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
@@ -99,6 +103,10 @@ jobs:
       - name: Check out code into the Go module directory
         uses: actions/checkout@v6
 
+      - uses: actions/setup-go@v6
+        with:
+          go-version-file: go.mod
+
       - name: Download dependencies
         run: go mod download