Update Keycloak to v26.3.4

[pankalog]

This PR updates Keycloak from v23 to v26.

One main reason I opened this PR is because my mTLS implementation would potentially need the TOKEN_EXCHANGE_STANDARD_V2 provider to perform token exchange between the manager and the authenticating user based off of their certificate's CN.

I'm not exactly sure what the validation steps are to merging this, please let me know if there's anything specific that I need to test!

[MartinaeyNL]

Added this to the projects board so we have visibility on this PR.

If you can share what the status of this PR is, if you need a review for example, would be great 👍

[pankalog]

@MartinaeyNL Yeah I wanted to do that, but I'm not sure what some steps would be to actually verify that the move to v26 is okay. Also, there's this issue and this issue, so I'm not sure if this is a PR we should spend time on or not.

[ebariaux]

This is indeed on-going work, I already had a branch with that change and #10 to track it but this is work @richturner is taking as a whole, updating the keycloak client in the manager and then upgrading the keycloak side version.
So I would close this as duplicate, maybe add a comment to the existing issues on why you also need this update for token exchange (but we haven't settle on using token exchange yet, so not blocking).