metropolis: use //version

This switches over the Metropolis codebase from the previous build
stamping system to the new //version library.

Change-Id: I8ecb0c3b4b19098e0026215444bae9b34060440e
Reviewed-on: https://review.monogon.dev/c/monogon/+/2333
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/version/gomod-generated-placeholder.go b/metropolis/version/gomod-generated-placeholder.go
new file mode 100644
index 0000000..f37d99d
--- /dev/null
+++ b/metropolis/version/gomod-generated-placeholder.go
@@ -0,0 +1 @@
+package version