Migrate substrate pallets to use `pallet!`

[gui1117]

Here is the list of pallet and their status, if you want to migrate one you can put your name after it to avoid duplicated work.
some help guidelines to do migration: https://crates.parity.io/frame_support/attr.pallet.html#upgrade-guidelines
some help guidelines to review a migration: https://crates.parity.io/frame_support/attr.pallet.html#checking-upgrade-guidelines

• pallet-assets @dvdplm Migrate assets pallet to new macros #7984
• pallet-atomic-swap @thiolliere
• pallet-aura
• pallet-authority-discovery
• pallet-authorship
• pallet-babe @shaunxw Migrate pallet-babe to pallet attribute macro. #8310
• pallet-balances @ascjones Migrate pallet-balances to pallet attribute macro #7936
• pallet-bounties @shamb0
• pallet-collective @koushiro
• pallet-contracts
• pallet-democracy
• pallet-elections (no need to to migrate except asked otherwise)
• pallet-elections-phragmen @kianenigma migrate pallet-elections-phragmen to attribute macros #8044
• pallet-example @thiolliere
• pallet-example-offchain-worker @thiolliere
• pallet-example-parallel @thiolliere
• pallet-grandpa @octol Migrate pallet-grandpa to attribute macros #8724
• pallet-identity
• pallet-im-online
• pallet-indices @ascjones Migrate pallet-indices to pallet! #8465
• pallet-lottery
• pallet-membership @koushiro
• pallet-metadata
• pallet-multisig
• pallet-nicks
• pallet-offences
• pallet-proxy @emostov Migrate pallet-proxy to pallet attribute macro #8365
• pallet-randomness-collective-flip
• pallet-recovery
• pallet-scheduler
• pallet-scored-pool
• pallet-session Migrate pallet-session to the new pallet macro #9796
• pallet-session-historical
• pallet-society
• pallet-staking @kianenigma
• pallet-sudo @ascjones Migrate pallet-sudo to pallet! #8448
• pallet-system @ascjones Migrate frame-system to pallet attribute macro #7898
• pallet-timestamp
• pallet-tips @koushiro
• pallet-transaction-payment @kianenigma
• pallet-treasury @shamb0
• pallet-utility @shaunxw Migrate pallet-utility to pallet attribute macro. #8326
• pallet-vesting @shaunxw Migrate pallet-vesting to pallet attribute macro. #8440
• node template Migrate pallet-template to pallet attribute macro #7981

[kianenigma]

pallet-executive

Is this a mistake?

pallet-elections

I wouldn't migrate this one until an external team or someone asks for it tbh. It is not really maintained.