Allow flushing of NpgsqlReadBuffer.Stream

[aromaa]

Changes the NpgsqlReadBuffer.Stream Flush to be nop to line up with the Stream.Flush{Async} documentation.

Fixes #4123

[vonzshik]

Well, that's a fun one. Definitely wasn't expecting anyone to call Flush on a read-only stream (least of all CryptoStream).
Anyway, could you please open an issue and add a test (you can put one in BugTests), which asserts that both Flush and FlushAsync don't throw?

[aromaa]

This was an interesting one indeed. And never even expected that the documentation mentions that flush should be valid on read-only streams, least that the BCL type relies on this!

[vonzshik]

Backported to 6.0.1 via 5cfe425, 5.0.12 via 40302e4.