Release mlr3learners 0.9.0

[be-marc]

Prepare for release:

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

Submit to CRAN:

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

Wait for CRAN...

• [ ] Accepted :tada:
• [ ] Finish & publish blog post
• [ ] Add link to blog post in pkgdown news menu
• [ ] usethis::use_github_release()
• [ ] usethis::use_dev_version(push = TRUE)
• [ ] Share on social media