grandpa: cleanup stale entries in set id session mapping (#13237)

* grandpa: cleanup stale entries in set id session mapping

* Update frame/grandpa/src/migrations.rs

Co-authored-by: Bastian Köcher <[email protected]>

* grandpa: remove unused import

* grandpa: migration off-by-one

* Update frame/grandpa/src/lib.rs

Co-authored-by: Anton <[email protected]>

* Update frame/grandpa/src/lib.rs

Co-authored-by: Anton <[email protected]>

* grandpa: MaxSetIdSessionEntries as u64

* node-template: fix MaxSetIdSessionEntries type

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Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Anton <[email protected]>