Allow validators to block and kick their nominator set. (#7930)

* Allow validators to block and kick their nominator set.

* migration

* Test

* Better migration

* Fixes

* cargo run --release --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_staking --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/staking/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* Update frame/staking/src/lib.rs

Co-authored-by: Shawn Tabrizi <[email protected]>

Co-authored-by: Parity Benchmarking Bot <[email protected]>
Co-authored-by: Shawn Tabrizi <[email protected]>