Fix plgx compilation issue caused by version drift.

[Fulgan]

plgx loading has been broken since the June 2024 dependency bump (issue #485 )

The plgx file works by shipping the project's source code plus all referenced DLLs; KeePass extracts everything to a temp folder and runs csc.exe against it on first load. That compile pass only sees what PlgxTool bundled — it does not read app.config, so the binding redirects that normally smooth over transitive dependency version mismatches at runtime are not applied.
Since c750b72 ("Updated dependencies") this has been a problem for several reasons that all reduce to the same shape:

• System.Net.Http is bundled at version 4.1.1.3 (from System.Net.Http 4.3.4's lib/net46), but Microsoft.Graph.Core 3.1.12 was compiled against 4.2.0.0 — csc raises CS1705. .NET Framework 4.7.2+ already includes 4.2.0.0 in the reference assemblies; the NuGet package isn't needed for net48.
• Google.Cloud.Storage.V1 4.10.0 was compiled against Google.Apis.Storage.v1 1.67.0.3365 while the project ships 1.68.0.3431. Same metadata-level mismatch: at runtime an app.config redirect would handle it, at plgx-compile time it doesn't and csc errors.

What this PR changes

• Removes the System.Net.Http 4.3.4 NuGet package and replaces the versioned reference with a bare . csc then resolves to the framework's 4.2.0.0, which is what Microsoft.Graph.Core expects.
• Bumps Google.Cloud.Storage.V1 4.10 → 4.11 (and the cascade: Google.Apis.Storage.v1 1.68.0.3431 → 1.68.0.3604, Google.Api.Gax and Google.Api.Gax.Rest 4.8 → 4.9) so every package's compile-time metadata agrees on the same 1.68.x baseline. No binding redirects required.
• Adds test-plgx.ps1, a script that builds Release, deploys the plgx to a portable KeePass install, runs KeePass --debug, and reports whether the plgx compiled cleanly. Default behavior is non-destructive; pass -Cleanup to also wipe %LOCALAPPDATA%\KeePass\PluginCache (useful when iterating on dependency fixes).

Edit: netstandard was actually already referenced. my mistake