[am11]

• Regex named blocks were already moved to Unicode 17.0 in Update Regex named blocks from Unicode 4.0 to Unicode 17.0 #120623 while the rest of the repo stayed on 16.0. This upgrades the remaining Unicode data (CoreLib, System.Text.Encodings.Web, Regex case equivalences and the native minipal casing table) straight to 18.0, the version now in dotnet/runtime-assets (Add Unicode 18.0.0 assets runtime-assets#715). Unicode 18.0 has no BMP block changes, so the named blocks table is unchanged.
• Bumps System.Private.Runtime.UnicodeData to 12.0.0-beta.26531.1 to pick up the 18.0 data ahead of [main] Update dependencies from dotnet/icu, dotnet/runtime-assets #133384.
• Converts the minipal UnicodeDataGenerator to a file-based app (similar to Replace Python CoreCLR test tooling with C# #135253) and fixes GenUnicodeProp and GenerateRegexCasingTable, which no longer built.

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