Verify compatibility w/ Pulsar 4.0.x

[onobc]

The latest Spring for Apache Pulsar version still uses Pulsar 3.3.2. There is no intention to switch the default to 4.0.0 in the 1.2.x line but we should add a compatibility test for Pulsar 4.0.0 and update our docs accordingly.

• Automated: Run samples w/ Pulsar 4.0.1 client libs
  ✅ (https://github.com/spring-projects/spring-pulsar/actions/runs/12331507494)
• Manual: Temp change to 4.0.1 in repo and run complete CI
• Manual: SB 3.4.x start.spring.io app w/ pulsar client override
• Automated: SB 3.4.x smoke tests
  ✅ Add Pulsar 4.0.x smoke test spring-boot#43532
• Update docs based on the outcomes from above