build: upgrade Go toolchain to 1.27 (#1946)

## Summary
- Bumps the Go toolchain used to build/test to 1.27:
`golang:1.27-alpine` in `Dockerfile` and `Dockerfile.go`, and
`go-version: '1.27'` in the three `actions/setup-go` steps in
`.github/workflows/deploy.yml`.
- Each module's `go.mod` `go` directive is intentionally left at `1.22`
— no 1.27-only language features are being adopted, and a 1.27 toolchain
builds a `go 1.22`-declared module without issue.

## Test plan
- [x] `go build ./...` + `go vet ./...` for all 13 modules
(`cmd/server`, `cmd/ingestor`, `cmd/migrate`, `cmd/decrypt`, 10
`internal/*` packages) under Go 1.27.0
- [x] `go test ./...` passes for `cmd/server`, `cmd/ingestor`,
`cmd/migrate`, `cmd/decrypt`
- [ ] `docker build` against the new `golang:1.27-alpine` base (Docker
wasn't available in the sandbox this change was prepared in — needs a
check in CI or locally)

.github/workflows/deploy.yml

@@ -103,10 +103,10 @@ jobs:
       - name: Clean Go module cache
         run: rm -rf ~/go/pkg/mod 2>/dev/null || true
 
-      - name: Set up Go 1.22
+      - name: Set up Go 1.27
         uses: actions/setup-go@v6
         with:
-          go-version: '1.22'
+          go-version: '1.27'
           cache-dependency-path: |
             cmd/server/go.sum
             cmd/ingestor/go.sum
@@ -343,10 +343,10 @@ jobs:
       - name: Clean Go module cache
         run: rm -rf ~/go/pkg/mod 2>/dev/null || true
 
-      - name: Set up Go 1.22
+      - name: Set up Go 1.27
         uses: actions/setup-go@v6
         with:
-          go-version: '1.22'
+          go-version: '1.27'
           cache-dependency-path: cmd/server/go.sum
 
       - name: Build Go server
@@ -758,10 +758,10 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v5
 
-      - name: Set up Go 1.22
+      - name: Set up Go 1.27
         uses: actions/setup-go@v6
         with:
-          go-version: '1.22'
+          go-version: '1.27'
 
       - name: Build corescope-decrypt (static, linux/amd64)
         run: |