Upgrade Flink connector to Flink 2.2 and Java 17

[nicolaspa1]

Summary

Minimal upgrade to support Apache Flink 2.2 and Java 17:

• flink-connector/pom.xml: bump Flink 1.17.2 → 2.2.0, Java 11 → 17
• flink-connector-gcp-pubsub/pom.xml: bump protoc 3.4.0 → 3.25.3
  (required for Flink 2.2 compatibility)
• PubSubSink.java: update createWriter signature from InitContext
  to WriterInitContext (API change in Flink 2.x / FLIP-143)

Motivation

Flink 2.x renamed Sink.InitContext to WriterInitContext as part of
the FLIP-143 unified sink API. Without this change the connector fails
to compile against Flink 2.2+.

This was validated end-to-end on GCP Dataproc 3.0 (Flink 2.2.0,
Java 17) consuming real Pub/Sub topics in a streaming pipeline
(cross-project subscriptions).

Testing

• Compiled and ran against Flink 2.2.0 on Dataproc 3.0-debian13
• Streaming job processed messages from Pub/Sub cross-project
  subscriptions successfully with PubSubSource (FLIP-27) and
  PubSubSink (FLIP-143)
• All existing tests pass

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