chore: Use internal secret for JWT key

[adwk67]

Description

Fixes #639

This change is non-breaking as the secret is created and used only "internally". The user does not have to create a secret themselves. The sample applies for the connections.secretKey that is part of the credentials secret used for DB connection information: this has been removed from all code (incl. examples, tests) and replaced with an internal secret. If the field is still provided it will be ignored.

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
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Author

• Changes are OpenShift compatible 🟢 tests
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added

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

[adwk67]

Release notes

The JWT key is now created by the the operator internally. The same applies to the key previously defined in the credentials secret under connections.secretKey: this change is non-breaking, as connections.secretKey will be ignored if supplied.

[maltesander]

LGTM!