build: move to Go 1.27.1

Go 1.27.1 (2026-09-01) carries fixes to the runtime, compiler, net/http,
encoding/json, os and database/sql. The go directive, the dependency check
script and the image build stage follow it; CI reads the version from
go.mod. The remaining golang.org/x modules the server requires are already
at their latest releases.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QwirCh4nsrJCQp2WaXoVLK
Signed-off-by: Feng Ruohang <rh@vonng.com>

buildscripts/checkdeps.sh

@@ -7,7 +7,7 @@ _init() {
 
 	## Minimum required versions for build dependencies
 	GIT_VERSION="1.0"
-	GO_VERSION="1.27.0"
+	GO_VERSION="1.27.1"
 	OSX_VERSION="10.8"
 	KNAME=$(uname -s)
 	ARCH=$(uname -m)