contrib: aom: update to 3.12.1

• Bug Fixes
  • b:396169342: Assertion `av1_is_subpelmv_in_range(&ms_params.mv_limits, start_mv)' failed.
  • b:401671154: typo in void init_src_params(...)
  • Coverity defect 323670: Uninitialized scalar variable in encode_with_and_without_superres()
  • cmake: bump minimum version to 3.16
  • cfl_ppc: fix subtract_average_vsx
  • Fix an incorrect index in av1_highbd_pixel_proj_error_neon