[R-package] rename internal callback functions

[mayer79]

Following #5018 (review),
changing the internal callback names:

• cb.early.stop() --> cb_early_stop()
• cb.print.evaluation() --> cb_print_evaluation()
• cb.record.evaluation() --> cb_record_evaluation()

[jameslamb]

Thanks for doing this!

I double-checked like this:

git grep -E "cb\."

and didn't find any other references.

[github-actions]

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