pmb.build.is_necessary: shorten warning and mention "pmbootstrap pull"
Add three patches working up to adding "pmbootstrap pull" in the warning that appears when binary packages are newer than the packages in pmaports. Get rid of the old link that wasn't that helpful (was written a long time ago).
Before:
WARNING: package postmarketos-artwork: aport version 4-r0 is lower than 5-r1 from the binary repository. 5-r1 will be used when installing postmarketos-artwork. See also: <https://postmarketos.org/warning-repo2>
After:
WARNING: about to install postmarketos-artwork 5-r1 (pmaports: 4-r0, consider 'pmbootstrap pull')