[FLINK-38123] Upgrade flink version in the autoscaling example

[kaygorodov]

What is the purpose of the change

The autoscaling example image is based on Flink 1.18 rather than 1.20.1.

Brief change log

• Upgrades Flink and Java version for the base image to align it with the version in the parent pom that are used to build the autoscaling example jar which are 1.20.1 (flink.version) and java 17 (maven.compiler.source)
• Removes the usage of deprecated Flink API switching to the new APIs (second commit)

Verifying this change

I manually build the image and tested the example locally (autoscaling-dynamic.yaml).

DOCKER_BUILDKIT=1 docker build . -t autoscaling-example:latest
apply -f autoscaling-dynamic.yaml 

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changes to the CustomResourceDescriptors: (no)
• Core observer or reconciler logic that is regularly executed: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)