[Followup] full migration to on_attach API mappings

[alex-courtis]

#1579

• document default mappings
• apply remove_keymaps
• legacy migration draft PR to discuss migration strategies for keymaps / on_attach #1464
• help shows on_attach mappings
• update help
• write migration guide
• ensure action="" applied to migration

[kyazdani42]

legacy migration (under the hood, automated): #1476
NEEDS: help shows on_attach mappings, document default mappings

[alex-courtis]

Tracked at #1579