Porting note: unimplemented `dangerous_instance` linter

[grunweg]

This issue is tracking all porting notes referring to the dangerous_instance linter from mathlib3: as this linter has not been ported to mathlib4 yet, the nolint entries from mathlib3 are commented. Porting notes look like

mathlib4/Mathlib/Algebra/Ring/CompTypeclasses.lean

Line 182 in b0aea61

 -- @[nolint dangerous_instance] Porting note: this linter is not implemented yet 

Steps to fix

• port the linter
• see which of the previous nolints are required: uncomment the necessary ones and delete the now-superfluous ones.
  Remove the porting note in either case.
• deal with any new errors in mathlib4
• land the linter and rejoice :-)