Bump required R version to >=4.0.0

Author: DavisVaughan

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 (>= 3.6.0)
+    R (>= 4.0.0)
 Imports:
     cli (>= 3.6.4),
     lifecycle (>= 1.0.4),

NEWS.md

@@ -13,6 +13,9 @@
 * Fixed `-Wdeprecated-literal-operator` reported by clang (#386,
   @MichaelChirico).
 
+* R >=4.0.0 is now required. This is consistent with the standards of the
+  tidyverse.
+
 # clock 0.7.2
 
 * Added a `diff()` method for time points and calendars to ensure that durations