CH-159 Keycloak upgrade configuration changes

[alxbrd]

Closes CH-159

Implemented solution

Update Keycloak to 26.2.1
Update events plugin configuration
Adjust template settings to ensure compatibility with new version of Kafka

How to test this PR

Create a typical application using harness-application then prepare deployment using harness-deployment. Deploy to a k8s instance and all KC related components should work as expected alongside KC 26.2.1.

Sanity checks:

• The pull request is explicitly linked to the relevant issue(s)
• The issue is well described: clearly states the problem and the general proposed solution(s)
• In this PR it is explicitly stated how to test the current change
• The labels in the issue set the scope and the type of issue (bug, feature, etc.)
• The relevant components are indicated in the issue (if any)
• All the automated test checks are passing
• All the linked issues are included in one Sprint
• All the linked issues are in the Review state
• All the linked issues are assigned

Breaking changes (select one):

• The present changes do not change the preexisting api in any way
• This PR and the issue are tagged as a breaking-change and the migration procedure is well described above

Possible deployment updates issues (select one):

• There is no reason why deployments based on CloudHarness may break after the current update
• This PR and the issue are tagged as alert:deployment

Test coverage (select one):

• Tests for the relevant cases are included in this pr
• The changes included in this pr are out of the current test coverage scope

Documentation (select one):

• The documentation has been updated to match the current changes
• The changes included in this PR are out of the current documentation scope

Nice to have (if relevant):

• Screenshots of the changes
• Explanatory video/animated gif

[filippomc]

Tested and works well. Requires more documentation on the update process, expecting that in a later PR