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.go

@@ -1,4 +1,4 @@
-FROM golang:1.22-alpine AS builder
+FROM golang:1.27-alpine AS builder
 
 RUN apk add --no-cache build-base