StringUtil: Remove JoinStrings

With 12 uses of `JoinStrings` in the codebase vs 36 uses of `fmt::join`, fmtlib's range adapter for string concatenation with delimiters is clearly the preferred option.

Source/Core/Core/Boot/Boot.cpp

@@ -15,6 +15,8 @@
 #include <utility>
 #include <vector>
 
+#include <fmt/ranges.h>
+
 #include "Common/Align.h"
 #include "Common/CommonPaths.h"
 #include "Common/CommonTypes.h"
@@ -90,7 +92,7 @@ static std::vector<std::string> ReadM3UFile(const std::string& m3u_path,
   if (!nonexistent.empty())
   {
     PanicAlertFmtT("Files specified in the M3U file \"{0}\" were not found:\n{1}", m3u_path,
-                   JoinStrings(nonexistent, "\n"));
+                   fmt::join(nonexistent, "\n"));
     return {};
   }