chore: set strum::Display for LogLevel to uppercase

[xeniape]

Description

Set strum::Display for LogLevel to uppercase and add warning.

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
- [ ] CRD changes approved
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] 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

[nightkr]

Where did this come up? Would it be more appropriate to not implement Display at all? I assume that the naming here was trying to align with something in the first place, are we drifting out of compat with that?