Pseudo header string reuse broken in .NET 6

[JamesNK]

Pseudo headers were filtered out of HTTP/2 and HTTP/3 requests with this PR - #36166

Unfortunately, the way they are filtered out - setting the header properties to default removes the bits AND the value - means string reuse no longer happens for those headers. e.g. :authority = localhost:80 allocates localhost:80 for every request.

I think the better way to filter these headers is to add a special method that clears their bits. We do something similar for some response headers:

aspnetcore/src/Servers/Kestrel/shared/KnownHeaders.cs

Lines 1165 to 1168 in 13a4608

	internal void ClearInvalidH2H3Headers()
	{{
	_bits &= ~{InvalidH2H3ResponseHeadersBits};
	}}

@Tratcher @halter73 @wtgodbe

[JamesNK]

The regression impact isn't large. An extra 4 small string allocations per request. We can decide whether it is worth fixing in .NET 6 once I have a fix.

FYI @adityamandaleeka