Move from slave to replica everywhere

Note that there is still one "slave" occurrence, but it's related to the S3 upload plugin and we cannot rename that property yet.

$ rg -i slave
vars/runTask.groovy
170:                            uploadFromSlave: true,

IMPORTANT: Please, don't merge this yet. I need to apply for a few changes in the CI jobs configuration at the same time this is merged. Just approving it is enough.