Release inlabru 2.6.0

[finnlindgren]

Prepare for release:

• [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] git flow release start 2.6.0
• [x] usethis::use_version('minor')
• [x] Push to github for checking; git flow release publish 2.6.0
• [x] devtools::check(remote = TRUE, manual = TRUE)
• [x] Local checking script for R-4.2.1, R-4.1.3 and R-devel
• [x] devtools::check_win_devel()
• [x] devtools::check_win_release()
• [x] devtools::check_win_oldrelease()
• [x] rhub::check_for_cran(env_vars=c("_R_CHECK_FORCE_SUGGESTS_"="false"))
• [x] revdepcheck::revdep_check(num_workers = 4)
• [x] Update cran-comments.md
• [x] git flow release finish 2.6.0
• [x] usethis::use_dev_version(), commit & push
• [x] git checkout stable, push and push tags

Submit to CRAN:

• [x] On the stable branch, devtools::submit_cran()
• [x] Approve email

Wait for CRAN...

• [x] Accepted :tada:
• [x] usethis::use_github_release()
• [x] Tweet