fix: allow uppercase characters in domain names

[xeniape]

Description

Implementation of #1022

According to RFC 1123 (which references RFC 952):

No distinction is made between upper and lower case.

This PR:

• Adjusts domain name regex and tests
• Moves code only used in tests into test module
• Removes name_is_dns_subdomain due to not being used in any other project

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
• CRD documentation for all fields, following the style guide.
• Integration tests passed (for non trivial changes)
• 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

[NickLarsenNZ]

LGTM

[NickLarsenNZ]

LGTM