Updates to survfitms models in {survival} v 3.6.4

[ddsjoberg]

What changes are proposed in this pull request?

• As of {survival} v3.6-4, the number censored are now returned as a matrix for multi-state models (i.e. competing risks models). The tidy_survfit() function has been updated to account for this new structure and the minimum version of {survival} has been increased to the latest version.

closes #199

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Reviewer Checklist (if item does not apply, mark as complete)

• Ensure all package dependencies are installed by running renv::install()
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• If a new function was added, function included in _pkgdown.yml
• If a bug was fixed, a unit test was added for the bug check
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
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• R CMD Check runs without errors, warnings, and notes
• usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged into master:

• Update NEWS.md with the changes from this pull request under the heading "# ggsurvfit (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end update (see NEWS.md for examples).
• Increment the version number using usethis::use_version(which = "dev")
• Run usethis::use_spell_check() again
• Approve Pull Request
• Merge the PR. Please use "Squash and merge".