Release valr 0.6.8

[kriemo]

Prepare for release:

• [x] PR #404
• [x] PR #406
• [x] git pull
• [x] Check current CRAN check results
• [x] Check if any deprecation processes should be advanced, as described in Gradual deprecation
• [x] Polish NEWS
• [x] devtools::build_readme()
• [x] urlchecker::url_check()
• [x] devtools::check(remote = TRUE, manual = TRUE)
• [x] devtools::check_win_devel()
• [ ] rhub::check_for_cran()
• [ ] rhub::check(platform = 'ubuntu-rchk')
• [ ] rhub::check_with_sanitizers()
• [x] revdepcheck::revdep_check(num_workers = 4)
• [x] Update cran-comments.md
• [x] git push

Submit to CRAN:

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

Wait for CRAN...

• [x] Accepted :tada:
• [x] git push
• [x] usethis::use_github_release()
• [x] usethis::use_dev_version()
• [x] git push