build: bump Go toolchain to 1.25.9 (#24293)

Bumps the Go toolchain from 1.25.8 to 1.25.9 across `go.mod`, the shared
setup-go action, and the dogfood image.
This keeps local builds, CI, and containerized workflows aligned on the
latest patch release, including the updated Go tarball checksum in the
Dockerfile.

go.mod

@@ -1,6 +1,6 @@
 module github.com/coder/coder/v2
 
-go 1.25.8
+go 1.25.9
 
 // Required until a v3 of chroma is created to lazily initialize all XML files.
 // None of our dependencies seem to use the registries anyways, so this