Bump version prefix to 2.6.0

Updated the VersionPrefix property in Directory.Build.props from 2.5.0 to 2.6.0 to prepare for the next project release.

Author: ds5678

Directory.Build.props

@@ -6,7 +6,7 @@
 		<IsAotCompatible>true</IsAotCompatible>
 		<CheckForOverflowUnderflow>true</CheckForOverflowUnderflow>
 
-		<VersionPrefix>2.5.0</VersionPrefix>
+		<VersionPrefix>2.6.0</VersionPrefix>
 		<Copyright>Copyright © ds5678</Copyright>
 		<Authors>ds5678</Authors>
 		<Company>AssetRipper</Company>