add_quantile() plateau bug fix

[KennethATaylor]

Got a little free time and fixed the bug I laid out in #270.

• Made updates to add_quantile() to fix the bug while ensuring no accidental breaking changes or unintentional issues with current test suite
• No new tests added since prior test suite already had some plots where plateaus were present. Snapshots were updated, but only the quantile annotations that were previously impacted by the bug were changed
• Updated NEWS.md

Closes #270

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

• Ensure all package dependencies are installed by running renv::install()
• PR branch has pulled the most recent updates from master branch. Ensure the pull request branch and your local version match and both have the latest updates from the master branch.
• 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.
• Overall code coverage remains >99.5%. Review coverage with withr::with_envvar(list(CI = TRUE), code = devtools::test_coverage()). Begin in a fresh R session without any packages loaded.
• 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".

[KennethATaylor]

Hey there, @ddsjoberg! Wanted to bump this bug fix from 3 weeks ago in case it got buried - let me know if anything is needed on my end

[ddsjoberg]

Hey there, @ddsjoberg! Wanted to bump this bug fix from 3 weeks ago in case it got buried - let me know if anything is needed on my end

Thank you for the PR @KennethATaylor ! Nothing from your end at the moment. I just haven't had the bandwidth recently. Hope to get to it soon!

[ddsjoberg]

Thank you for the PR! VERY much appreciated!