Release mlr3learners 0.6.0

[be-marc]

Prepare for release:

• [x] git pull
• [x] Check current CRAN check results
• [x] Polish NEWS
• [x] urlchecker::url_check()
• [x] devtools::check(remote = TRUE, manual = TRUE)
• [x] devtools::check_win_devel()
• [x] revdepcheck::revdep_check(num_workers = 4)
• [x] Update cran-comments.md
• [x] git push
• [x] Draft blog post

Submit to CRAN:

• [x] usethis::use_version('minor')
• [x] devtools::submit_cran()
• [x] Approve email

Wait for CRAN...

• [x] Accepted :tada:
• [ ] Add preemptive link to blog post in pkgdown news menu
• [ ] usethis::use_github_release()
• [ ] usethis::use_dev_version(push = TRUE)
• [ ] Finish blog post
• [ ] Tweet