Remove type keyword from layout error report

[frangio]

In the error report we keep the struct/contract keywords. Yet, in the retyped annotations we expect the type without those keywords. A user might copy the type from the error and put it in the retype annotation and it would not work (see also #727).

See struct V2.ProposalCore below:

Error: New storage layout is incompatible

contracts/Test.sol:36: Upgraded `_proposals` to an incompatible type
  - In mapping(uint256 => struct V2.ProposalCore)
    - Bad upgrade from struct V1.ProposalCore to struct V2.ProposalCore

I believe we should also remove the keywords from the error messages, as a result the types will look more like Solidity to the user and they will also be copy-pastable into the retyped annotation.

As a side note, the function should probably be renamed:

openzeppelin-upgrades/packages/core/src/storage/compare.ts

Lines 411 to 415 in 1bca9c3

	export function stripContractSubstrings(label?: string) {
	if (label !== undefined) {
	return label.replace(/\b(contract|struct|enum) /g, '');
	}
	}