OSSM 2.1 Upgrade: Check previous release Mixer enablement before upgrading
OSSM 2.1 deprecated Mixer for istio policy and telemetry features.

Acceptance Criteria:

• If an SMCP with version 2.1 is created or updated and has mixer enabled, the validation should fail and warn the user about Mixer being removed
• (optional) if possible, show a warning when a 2.0 SMCP is created or updated that mixer is deprecated
• docs: release notes for 2.1 state that mixer must be disabled before upgrading