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)

Dockerfile

@@ -3,7 +3,7 @@
 # BUILDPLATFORM is auto-set by buildx; default to linux/amd64 so plain
 # `docker build` (without buildx) doesn't fail on an empty platform string.
 ARG BUILDPLATFORM=linux/amd64
-FROM --platform=$BUILDPLATFORM golang:1.22-alpine AS builder
+FROM --platform=$BUILDPLATFORM golang:1.27-alpine AS builder
 
 ARG APP_VERSION=unknown
 ARG GIT_COMMIT=unknown