chore: simplify codebase

[maximiliantech]

What this PR does / why we need it:
This PR adds:

• code comments for interfaces and their implementation
• introducing feature to read kubeconfigs properly in production and dev environments
• removes the AccessRequest API and instead using the implementation from https://github.com/openmcp-project/openmcp-operator/blob/main/api/clusters/v1alpha1/accessrequest_types.go
• Keeping the Cluster API implementation since the openmcp-operator is not yet using the correct underlying structs
• go.mod: updates go version to 1.24.3
• go.mod: updates direct and indirect dependencies
• refactor the pkg/smartrequeue package
• add more unit tests in the pkg/... directories
• remove end-to-end test suite as this is not needed right now

Which issue(s) this PR fixes:
Related issue: openmcp-project/backlog#11
First attempt to fix #26

Special notes for your reviewer:

Release note:

Upgrading go.mod version, direct and indirect dependencies

Introducing feature to read kubeconfigs properly in production and dev environments

Refactor smartrequeue package for better legibility and add more unit tests 

[reshnm]

Overall LGTM.
Just one question regarding the type definitions.

[reshnm]

Is there a reason we don't use the common definition from openmcp-operator instead of re-defining things here?

[maximiliantech]

Yes, I will do that step by step with the next PRs. Right now, the Cluster resource in the openmcp-operator does not support Conditions from metav1.[]Condition yet. It is a temporary solution until the openmcp-operator fully incorporates our latest API decisions.