Bump k8s deps to v1.28, kapp-ctrl to v0.49 and flux2 to >=v2.0.0

[antgamdia]

Description of the change

This PR is bumping up some dependencies together, as they are somehow coupled. Besides, it performs some changes on the Flux resources (including the changes performed in #7132 from @kvaps; thank you for the PR!) to make it compatible with their v2.0.0 release onwards.

As pointed out in #7132 (comment), some additional changes were required, namely:

• Effectively use latest Flux version in CI, for identifying the issue.
• Replace PollInfinite with PollUntilContextCancel bc of a k8s deprecation.
• Rename fluxcd imports to sourcev1beta2 to clearly identify which version we are using.
• Add TODOs for using CertSecretRef in the future (upcoming deprecation, but it will be addressed in a separate PR).
• Minor change in method signature because of the kapp-ctrl upgrade.

Benefits

We'll be able again to get dependabot's upgrades in these dependencies and, more importantly, Kubeapps will work again with latest FluxCD releases.

Possible drawbacks

Conversely, Kubeapps will likely fail with Flux2 version prior to 2.0.0. See more information and migration guide at: https://github.com/fluxcd/flux2/releases/tag/v2.0.0

Applicable issues

• fixes Empty catalog using fluxcd #6863

Additional information

N/A

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[antgamdia]

After an IRL test, it seems the catalog is now fetched properly: