chore: take the version from VERSION.txt only (0.0.3)

VERSION.txt is embedded into the server and the agent, so a plain
go build reports the real version and build.sh no longer injects it.

version.go

@@ -0,0 +1,14 @@
+// Package lpra holds what the server and the agent share at build time.
+package lpra
+
+import (
+	_ "embed"
+	"strings"
+)
+
+//go:embed VERSION.txt
+var versionFile string
+
+// Version is the release version of every binary in this module. VERSION.txt
+// is the only place it is defined.
+var Version = strings.TrimSpace(versionFile)