[JENKINS-68471] Prepare SAML for removal of JAXB and Java 11 requirement

[basil]

See JENKINS-68471. This plugin depends on org.pac4j:pac4j-saml, which depends on org.pac4j:pac4j-saml, which depends on org.opensaml:opensaml-security-api, which depends on org.apache.santuario:xmlsec, which depends on JAXB:

org.jenkins-ci.plugins:saml:hpi:2.999999-SNAPSHOT
+- org.pac4j:pac4j-saml:jar:3.9.0:compile
|  +- org.opensaml:opensaml-security-api:jar:3.4.3:compile
|  |  +- org.apache.santuario:xmlsec:jar:2.3.0:compile
|  |  |  +- JAXB

But when running on Java 9+, JAXB is not included on the classpath by default. The only way for a plugin to have access to JAXB when running on Jenkins 2.164 or later on Java 11 is to declare a plugin-to-plugin dependency on JAXB API plugin (recommended) or to embed JAXB into its .jpi file. This PR does the former to ensure that SAML always has access to JAXB on its classpath. CC @kuisathaverat

[kuisathaverat]

It is weird I did not detect it on any of the issues I have troubleshooting at https://github.com/kuisathaverat/jenkins-issues it is been a while since I use JDK 11, in any case, seems fair to include the dependency.