Prepare for Spring Security 6.3

[jzheaux]

Similar to #13416

• Update SecurityNamespaceHandler to require 6.3
• Add spring-security-6.3.rnc and spring-security-6.3.xsd
• Update spring.schemas; add lines for spring-security-6.3.xsd and change the setting for spring-security.xsd to point to spring-security-6.3.xsd
• Point spring-security.xsd symlink to spring-security-6.3.xsd
• Update XsdDocumentedTests to point to 6.3
• Increment schema count in XsdDocumentedTests
• Update SpringSecurityCoreVersion to 630L
• 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 6.3 schema
• Update What's New to an empty 6.3 section
• Update prepare-forward-merge to point to 6.3.x
• Update gradle.properties version to 6.3.0-SNAPSHOT
• Update security.tld tlib-version to 6.3
• Update Dependabot configuration
• Update release-scheduler.yml
• Update samplesBranch in gradle.properties