Bump minimal R version to >=3.6

We already have this transitively through vctrs since glue requires 3.6. This also lets us be absolutely sure that `STRING_PTR_RO()` is available.

DESCRIPTION

@@ -15,7 +15,7 @@ License: MIT + file LICENSE
 URL: https://clock.r-lib.org, https://github.com/r-lib/clock
 BugReports: https://github.com/r-lib/clock/issues
 Depends:
-    R (>= 3.5.0)
+    R (>= 3.6.0)
 Imports:
     cli (>= 3.6.1),
     lifecycle (>= 1.0.3),