[26.6] Do not allow updates on the realm-management client - #49956

@pedroigor - this PR now has conflicts, please resolve.

@pedroigor - I pushed a change to move it to the next upcoming patch release. Should be fine now; waiting for the build to complete.

Unreported flaky test detected

If the flaky tests below are affected by the changes, please review and update the changes accordingly. Otherwise, a maintainer should report the flaky tests prior to merging the PR.

org.keycloak.testsuite.broker.KcOidcBrokerTest#loginWithExistingUserWithBruteForceEnabled

Keycloak CI - Java Distribution IT (windows-latest - temurin - 21)

org.openqa.selenium.TimeoutException: 
Expected condition failed: waiting for value to contain (ignoring case) "sign in to". Current value: "AUTH_RESPONSE" (tried for 5 second(s) with 500 milliseconds interval)
Build info: version: '4.28.1', revision: '73f5ad48a2'
System info: os.name: 'Windows Server 2025', os.arch: 'amd64', os.version: '10.0', java.version: '21.0.11'
Driver info: org.jboss.arquillian.drone.webdriver.htmlunit.DroneHtmlUnitDriver_ByGraphene
...

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