chore: Upgrade stackable-operator to 0.121.0

[siegfriedweber]

Description

Upgrade stackable-operator to version 0.121.0 and replace the unmaintained serde_yaml crate with yaml_serde.

Part of stackabletech/issues#898

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• Helm chart can be installed and deployed operator works
• Integration test passed locally:

  --- PASS: kuttl (305.86s)
      --- PASS: kuttl/harness (0.00s)
          --- PASS: kuttl/harness/smoke_opensearch-3.8.0_opensearch_home-_stackable_opensearch_server-use-tls-true (305.84s)
  

• 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.
• 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
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker