Enhancements to Pega deployment

What's Changed

• ISSUE-680 Fix OAuth Client Secret env variable by @maracle6 in #681
• US-512975 Support custom backendConfig for GKE by @MadhuriArugula in #747
• [BUG-867838] Platform/Search Docker image deprecation by @kingakowalska1 in #743
• Issue-635: Pod level security context configuration for Pega chart & Hazelcast sub chart by @kishorv10 in #750
• Syncing values.yaml with values-large.yaml by @Evan1oconto in #745

Full Changelog: v3.19.0...v3.20.0