Drop support for R 3.6 and remove dead code

[DavisVaughan]

• Dropped support for R 3.6, as it is getting too challenging to try and continue to run CI for, and it is past our tidyverse EOL date by >6 months at this point.
• Removed as much dead code as possible. Anything that relied on R_Version(3, 5, 0), R_Version(3, 6, 0), or >= R_Version(4, 0, 0) has been removed because R >= 4.0.0 is now required.
• Kept altrep.hpp and HAS_ALTREP because arrow uses it
• Kept HAS_UNWIND_PROTECT because arrow and systemfonts use it