upgrade macos CI from 14 to latest

[tdhock]

Closes #94

[tdhock]

Preview

[tdhock]

* checking tests ...
  Running ‘testthat.R’ [161s/178s]
 [162s/179s] ERROR
Running the tests in ‘tests/testthat.R’ failed.
Last 13 lines of output:
   1. └─atime::atime_pkg_test_info(pkg.dir) at test-versions.R:136:3
  
  ══ Failed tests ════════════════════════════════════════════════════════════════
  ── Failure ('test-versions.R:42:3'): atime_pkg produces tests_all_facet.png and tests_preview_facet.png on atime-test-funs ──
  Expected every element of `issues.dt$issue` to match regexp "slower".
  Actual text:
  ✔ │ HEAD=atime-test-funs much slower
  ✔ │ HEAD=atime-test-funs much slower
  ✔ │ HEAD=atime-test-funs much slower
  ✖ │ HEAD=atime-test-funs much faster
  
  [ FAIL 1 | WARN 3 | SKIP 0 | PASS 100 ]
  Error:
  ! Test failures.
  Execution halted
* checking for unstated dependencies in vignettes ... OK
* checking package vignettes ... OK
* checking re-building of vignette outputs ... [70s/72s] OK
* checking for non-standard things in the check directory ... OK
* checking for detritus in the temp directory ... OK
* DONE
Status: 1 ERROR