ls(1): Update all POSIX references and correct the STANDARDS section

• Update the POSIX references for non-standard ls(1) options
• Simplify the STANDARDS section by mention both supported POSIX versions

Reported by: hrs
Reviewed by: hrs, bcr
Approved by: hrs, bcr
MFC after: 3 days
X-MFC-With: r364449
Differential Revision: https://reviews.freebsd.org/D26210