refactor(x/staking): Migrate UnbondingDelegation to collections

[likhita-809]

Description

Closes: #16381

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[facundomedica]

This migration doesn't seem complete as SetUnbondingDelegation still uses store.Set.

BTW here's the diff test for this (add key *storetypes.KVStoreKey to the suite and add the missing imports), the hash is from main:

func (s *KeeperTestSuite) TestDiffCollsMigration() {
	s.SetupTest()

	delAddrs, valAddrs := createValAddrs(100)
	err := sdktestutil.DiffCollectionsMigration(
		s.ctx,
		s.key,
		100,
		func(i int64) {
			ubd := stakingtypes.UnbondingDelegation{
				DelegatorAddress: delAddrs[i].String(),
				ValidatorAddress: valAddrs[i].String(),
				Entries: []stakingtypes.UnbondingDelegationEntry{
					{
						CreationHeight: i,
						CompletionTime: time.Unix(i, 0).UTC(),
						Balance:        math.NewInt(i),
						UnbondingId:    uint64(i),
					},
				},
			}
			err := s.stakingKeeper.SetUnbondingDelegation(s.ctx, ubd)
			s.Require().NoError(err)
		},
		"d03ca412f3f6849b5148a2ca49ac2555f65f90b7fab6a289575ed337f15c0f4b",
	)

	s.Require().NoError(err)
}

[tac0turtle]

few places the global bech32 is being used, can we revert this to the address codec

[atheeshp]

overall lgtm

[atheeshp]

lgtm