Fix PgVector ReloadTypesAsync compatibility with newer Npgsql

[WyRainBow]

Summary

• fix PgVector runtime compatibility when Npgsql exposes ReloadTypesAsync(CancellationToken) instead of the no-arg overload
• avoid direct binding to a single method signature in PostgresCollection
• use runtime-compatible method resolution in PostgresUtils

Validation

• dotnet test dotnet/test/VectorData/PgVector.UnitTests/PgVector.UnitTests.csproj

[WyRainBow]

@microsoft-github-policy-service agree

[roji]

Thanks. This seems to be because of npgsql/npgsql#5919, where Npgsql 9.0 made a binary breaking change by adding the cancellation token parameter; the user is presumably trying to use the MEVD provider with Npgsql 9/10, but it was compiled against Npgsql 8.0.

Rather than the elaborate caching reflection-based approach, we should just have the old overload added back at the Npgsql level. I've submitted npgsql/npgsql#6522 to do this, and will close the fix on this side (if there's some issue on the Npgsql side we'll revisit on the MEVD side).

The even better way to handle this is to have the provider use the newest Npgsql (10.0) when targeting modern .NET, and only use old 8.0 when targeting netstandard2.0/netfx - I've done that in #13724.