fix: update documentation and test to use 2.6.1

[dervoeti]

Description

I forgot to update tests and docs to the latest HBase version in #627

I removed the line with jmx_prometheus_javaagent.jar in the test_construct_jvm_argument_overrides test since the JMX exporter is not needed for HBase 2.6 and the test would fail otherwise.

Definition of Done Checklist

# Author
- [x] Changes are OpenShift compatible
- [x] CRD changes approved
- [x] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide).
- [x] Helm chart can be installed and deployed operator works
- [x] Integration tests passed (for non trivial changes)
- [x] Changes need to be "offline" compatible

# Reviewer
- [ ] Code contains useful comments
- [ ] Code contains useful logging statements
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide).
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added
- [ ] [Roadmap](https://github.com/orgs/stackabletech/projects/25/views/1) has been updated

[razvan]

lgtm!

[razvan]

What is unexpected is this check really. I wonder why it's there.