[FLINK-36129][autoscaler] Autoscaler is compatible with Flink 1.20

[1996fanrui]

What is the purpose of the change

AggregatedMetric added the skew field in Flink 1.20, it caused the AggregatedMetric of old version cannot parse the AggregatedMetric of 1.20.

Following is the root exception:

Caused by: org.apache.flink.shaded.jackson2.com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "skew" (class org.apache.flink.runtime.rest.messages.job.metrics.AggregatedMetric), not marked as ignorable (5 known properties: "min", "id", "max", "avg", "sum"])
at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: java.util.ArrayList[0]->org.apache.flink.runtime.rest.messages.job.metrics.AggregatedMetric["skew"])

Brief change log

• [FLINK-36129] Supporting Flink 1.20 and removing Flink 1.15
  • Here is the CI result of first commit.
  • We can see Unrecognized field "skew" related error, it works after the second commit.
• [FLINK-36129][autoscaler] Autoscaler is compatible with Flink 1.20
  • Copy AggregatedMetric to this repo, and removing it after upgrading maven dependency to 1.20.
  • AggregatedMetric is copied into 3 modules: flink-autoscaler, flink-kubernetes-operator and flink-autoscaler-standalone
  • Copying it to flink-autoscaler, because flink-autoscaler is using AggregatedMetric.
  • Copying it to flink-kubernetes-operator and flink-autoscaler-standalone, because we hope the copied AggregatedMetric to replace the AggregatedMetric of maven dependency.

Verifying this change

Added the CI to run with 1.20.

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

[1996fanrui]

Hey @gyfora , I found autoscaler doesn't work with flink 1.20. This PR did 2 changes:

1. Operator supports flink 1.20, and enable CI for 1.20 (disable CI for 1.15)
2. Fix autoscaler compatibility issue with 1.20

I explained them in detail in the PR description.

[1996fanrui]

Thanks @gyfora for the quick review, I will merge it if no more comment within 24 hours.