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

version.txt

@@ -0,0 +1 @@
+0.1