Prepare for Spring Security 6.4

[marcusdacoregio]

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