Remove COMPLUS_* environment variable references (#5714)

Minimum supported runtime is now .NET 8, so `COMPLUS_*` env var
fallbacks are unnecessary — all config uses `DOTNET_*`.

- **`ProcessRunner.WithRuntimeConfiguration`**: Remove `COMPlus_`
fallback (the existing comment explicitly called for this)
- **`documentation/FAQ.md`**: Remove stale note about .NET 6 requiring
`COMPlus_` prefix

Files in `src/shared/` (`clrconfignocache.h`, `resource.h`) are synced
from dotnet/runtime and left unchanged — `BFA_BAD_COMPLUS_SIG` refers to
COM+ metadata signatures, not env vars.

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documentation/FAQ.md

@@ -73,7 +73,7 @@ Frequently Asked Questions
 
     If that doesn't work, try using the `setclrpath <directory>` command with a directory that contains the matching version of the DAC module. This is useful for private runtimes or debug builds that haven't been published to our symbol servers.
 
-    If this is a dump, the problem could also be that the dump is missing some memory required by SOS. Try generating a "full" dump (the default with `dotnet-dump collect` without a `--type` option) or add setting the crash dump generation (createdump) environment variable `DOTNET_DbgMiniDumpType=4` (.NET 6 and below require `COMPlus_` prefix instead of `DOTNET_`). For more details on crash dump generation see [here](https://docs.microsoft.com/en-us/dotnet/core/diagnostics/dumps#collecting-dumps-on-crash).
+    If this is a dump, the problem could also be that the dump is missing some memory required by SOS. Try generating a "full" dump (the default with `dotnet-dump collect` without a `--type` option) or add setting the crash dump generation (createdump) environment variable `DOTNET_DbgMiniDumpType=4`. For more details on crash dump generation see [here](https://docs.microsoft.com/en-us/dotnet/core/diagnostics/dumps#collecting-dumps-on-crash).
 
 * If you receive this error message executing a SOS command:
     ```

src/Microsoft.Diagnostics.TestHelpers/ProcessRunner.cs

@@ -122,10 +122,8 @@ public ProcessRunner RemoveEnvironmentVariable(string key)
             return this;
         }
 
-        // Remove COMPlus_ fallback once minimum supported runtime is .NET 8
         public ProcessRunner WithRuntimeConfiguration(string key, string value) =>
-            WithEnvironmentVariable($"DOTNET_{key}", value)
-            .WithEnvironmentVariable($"COMPlus_{key}", value);
+            WithEnvironmentVariable($"DOTNET_{key}", value);
 
         public ProcessRunner WithEnvironmentVariable(string key, string value)
         {