Release bibliometrix 5.2.1

[massimoaria]

Prepare for release:

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

Submit to CRAN:

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

Wait for CRAN...

• [ ] Accepted :tada:
• [ ] usethis::use_github_release()
• [ ] usethis::use_dev_version(push = TRUE)