allow signed migration

[xlc]

will include this into 2260 runtime

[zjb0807]

It's better to adjust the deposit

pub MigrationSignedDepositPerItem: Balance = dollar(KAR);
pub MigrationSignedDepositBase: Balance = dollar(KAR);

https://github.com/polkadot-fellows/runtimes/blob/cf15699e1527f3bf4c76abd0287e62721bee428c/relay/polkadot/src/lib.rs#L1540-L1541

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.94%. Comparing base (4c725dd) to head (4ec9caa).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2803      +/-   ##
==========================================
- Coverage   65.19%   64.94%   -0.25%     
==========================================
  Files          67       65       -2     
  Lines        9512     8635     -877     
==========================================
- Hits         6201     5608     -593     
+ Misses       3311     3027     -284     

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[xlc]

only a single account can trigger migration so high deposit is not required