Simplify csproj configuration - Set x64 as default platform and unify LangVersion

[SamLJackson2]

The current [buddy.csproj] has unnecessary configuration complexity:

1. AnyCPU as default: Heartopia is a 64-bit game using MelonLoader (net6), so x64 is the only target. AnyCPU configs are unused and confusing.
2. LangVersion mismatch: Global config specifies LangVersion 8.0, but x64 platform configs explicitly override to 7.3. The codebase uses only C# 7.3 features (no nullable refs, records, init-only properties, or switch expressions).
3. Verbose build commands: Users must type dotnet build buddy.csproj -c Debug -p:Platform=x64 instead of just dotnet build.

Solution

1. Change default platform from AnyCPU → x64
2. Unify LangVersion to 7.3 globally (verified via codebase scan)
3. Keep minimal AnyCPU configs as transparent proxies that output to bin\x64* and set PlatformTarget=x64
   This allows dotnet build to work without flags while maintaining x64 output
4. Remove redundant LangVersion overrides from platform-specific configs

In my testing
dotnet build → builds Debug to bin\x64\Debug
dotnet build -c Release → builds Release to bin\x64\Release
No compilation errors (12 warnings, all pre-existing)

If theres a reason it is the way that it is please let me know, otherwise i can make a PR if you have the same issue.

[SamLJackson2]

tldr; this would allow building to be done with "dotnet build" instead of "dotnet build buddy.csproj -c Debug -p:Platform=x64"
But i wouldnt dare to push a PR of editing the csproj without at least asking about why it exists this way.

[Rayyy2]

it was modified by nightshift. i can't build new dll. im not at home in 4 days. since yesterday
gonna be busy. maybe samjack can make discord server.

[SamLJackson2]

i am also the nightshift account, i was just accidentally logged into the wrong account when i pushed a few commits 💀

everything in this file has been untouched since you updated the source code.