Allow enabling the servant build flag with older versions of stm

Allowing building with ghc 9.0.2 (debian stable).

Updated patch covering all uses of writeTMVar.

Utility/STM.hs

@@ -0,0 +1,23 @@
+{- support for old versions of the stm package
+ -
+ - Copyright 2024 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+{-# LANGUAGE CPP #-}
+{-# OPTIONS_GHC -fno-warn-tabs #-}
+
+module Utility.STM (
+	module Control.Concurrent.STM,
+#if ! MIN_VERSION_stm(2,5,1)
+	writeTMVar
+#endif
+) where
+
+import Control.Concurrent.STM
+
+#if ! MIN_VERSION_stm(2,5,1)
+writeTMVar :: TMVar t -> t -> STM ()
+writeTMVar t new = tryTakeTMVar t >> putTMVar t new
+#endif