Similar to https://github.com/spring-projects/spring-security/issues/14210

  • Update SecurityNamespaceHandler to require 6.4
  • Add spring-security-6.4.rnc and spring-security-6.4.xsd
  • Update spring.schemas; add lines for spring-security-6.4.xsd and change the setting for spring-security.xsd to point to spring-security-6.4.xsd
  • Point spring-security.xsd symlink to spring-security-6.4.xsd
  • Update XsdDocumentedTests to point to 6.4
  • Increment schema count in XsdDocumentedTests
  • Update RNC link in docs/modules/ROOT/pages/servlet/appendix/namespace/index.adoc
  • Change config/src/testresources/org/springframework/security/config/method-security.xml to reference the 6.4 schema
  • Update What's New to an empty 6.4 section
  • Update prepare-forward-merge to point to 6.4.x
  • Update gradle.properties version to 6.4.0-SNAPSHOT
  • Update security.tld tlib-version to 6.4
  • Update Dependabot configuration
  • Update release-scheduler.yml
  • Update samplesBranch in gradle.properties
  • Update Test Against Snapshots in continuous-integration.yml to use new snapshot versions if needed