Rename usage of the term 'slave' to 'agent'

[mkemmerz]

https://issues.jenkins.io/browse/JENKINS-42844

The wording of "slave" has been changed to the term "agent". This was done for classes, methods, config and JAR files.
No bug fixing was done and no new feature was introduced.

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[oleg-nenashev]

It is a good start but it will lead to regressions on older agents that use "slave.xml". Ideally there should be migration logic.

Also some unrestricted classes are renamed, it is likely to caused regressions in downstream, e.g. in https://github.com/jenkinsci/graphql-server-plugin/blob/6fffb7ecb178ae4804ce32bee91185f64c4abaa6/src/test/java/io/jenkins/plugins/graphql/MockClassUtils.java . It is not a big deal since it is unit test but there might be similar issues in 3rd party plugins