Rename version to version.txt

<version> is a standard header with C++20 which could cause issues if a library checks it exists then imports it

Because we have the root of this repo in the include path, it'd see that <version> exists (with, e.g., __has_include), and then try to include it as a header

But because it's just a file that says 0.1, this would fail

This happens with libpqxx 7

flake.nix

@@ -6,7 +6,7 @@
   outputs = { self, nixpkgs, nix }:
     let
 
-      version = "${builtins.readFile ./version}.${builtins.substring 0 8 self.lastModifiedDate}.${self.shortRev or "DIRTY"}";
+      version = "${builtins.readFile ./version.txt}.${builtins.substring 0 8 self.lastModifiedDate}.${self.shortRev or "DIRTY"}";
 
       pkgs = import nixpkgs {
         system = "x86_64-linux";