Add a note on Release Notes 8.12 of .NET Client

[rodrigomadalozzo]

The InMemoryTransportClient was renamed to InMemoryRequestInvoker in this pull request: Attempt at renames. This name change occurred in the 0.4.18 release, which was included in the 8.12 stack version. Reference: https://github.com/elastic/elastic-transport-net/blob/0.4.18/benchmarks/Elastic.Transport.Benchmarks/TransportBenchmarks.cs#L18.

The changes to elastic-transport-net should be documented in the release notes, as they affect users utilizing transport directly.

This should be documented as a new Release notes v8.12.0 entry section on https://www.elastic.co/guide/en/elasticsearch/client/net-api/current/release-notes.html