🔧 Replaces ineffective merge drivers with EOL normalisation

merge=upstream-wins named a driver that is not built in and is not
defined in any git config, so git silently fell back to the default
3-way merge. Custom merge drivers live in .git/config and cannot be
distributed via .gitattributes, so downstream clones never had it.

merge=union on Directory.Packages.props silently produced duplicate
PackageVersion entries when both sides changed the same Include,
which NuGet rejects at build time with no conflict to review.

No tracked blob is stored with CRLF, so text=auto is a no-op for
existing content and only guards future contributions.

.gitattributes

@@ -1,2 +1,3 @@
-Directory.Packages.props merge=union
+* text=auto
-Directory.Packages.template.props merge=upstream-wins
+*.png binary
+*.cs diff=csharp