Fix macOS 26.x version check to allow subversions (e.g., 26.0.1) in is_macos_r_supported

[iamudyavar]

Trying to run macrtools::macos_rtools_install() on macOS 26.0.1 yields the below error:

Error in `macrtools::macos_rtools_install()`:
! macrtools: Your macOS version "26.0.1" is not supported.
macrtools: Supported versions: macOS High Sierra (10.13) through macOS Tahoe
(26.x).
Run `rlang::last_trace()` to see where the error occurred.

What was fixed:
The upper bound in is_macos_r_supported() was changed from "26.0" to "27.0", permitting all 26.x.y macOS versions (such as 26.0.1).

[coatless]

Thanks!