v0.11.20: Update PPR

What's Changed

• Update PPR + fix documentation by @pablohoch in #449

Update PPR to the latest version, including the following updates (PPR data needs to be imported again to benefit from the improvements):

• Fix stairs direction ppr#58
• Crossing path improvements ppr#60
• Create fewer crossings at nodes ppr#61
• Wheelchair profile: remove elevation cost ppr#62

Also fixes #448 (mistake in an example in the PPR api docs).

Full Changelog: v0.11.19...v0.11.20