Rename burn to withdraw in pallet and bridge code

[DylanVerstraete]

• [x] change burn to withdraw in pallet code
• [x] do migration for renaming a storage map
• [x] update Markdown files
• [x] update substrate go client
  • https://github.com/threefoldtech/substrate-client/issues/63
• [x] update on tfchain repo
  • https://github.com/threefoldtech/tfchain/issues/539
• [ ] ~~update graphql (?)~~ we will stop tracking bridge on graphql