Extensions: betterness for static members should only consider type of extension parameter, not other modifiers #79171
Description
From LDM 2025-06-23, we need to adjust extension resolution on type receivers, so they are not affected by irrelevant aspects of the extension parameter (such as in vs. by-value differences).
https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-06-23.md#lookup
Relates to test plan #76130