postgresqlPackages.omnigres: unpin clang_18

we are having trouble ascertaining *why* this this was originally pinned
at all, given that this was added in the init of omnigres and the init
pr does not seem to offer more clarity on why this is. given that it
still builds with an unpinned clang, we simply unpin and let it use the
default version of clang, which helps reduce compiler version
proliferation and prepared for the drop of llvm 18.

frankly, we are not entirely sure why this has a clang dependency at
all, and it seems to build just fine without it. if this is supposed
to be built with clang, it seems that it should be using `clangStdenv`
or `llvmPackages_NN.stdenv` in some form, but that does not seem to be
the case. however, without further domain expertise we're not inclined
to just remove the dependency. we would be more than happy for someone
more familiar to tell us that this can simply be dropped.

pkgs/servers/sql/postgresql/ext/omnigres.nix

@@ -1,6 +1,6 @@
 {
   brotli,
-  clang_18,
+  clang,
   cmake,
   fetchFromGitHub,
   flex,
@@ -58,7 +58,7 @@ postgresqlBuildExtension (finalAttrs: {
   strictDeps = true;
 
   nativeBuildInputs = [
-    clang_18
+    clang
     cmake
     flex
     netcat