fix: fixes vuln in Multisig Plugin

[mathewmeconry]

Description

Code4rena finding: code-423n4/2023-03-aragon-findings#140

When the Multisig settings and addresses are updated and in the same block another proposal gets created an attacker could trigger a race condition:

Consider the addresses 0x01, 0x02, 0x03 to be a member of the Multisig, and the minApprovals is set to 2 during block 1.
Now 0x03 is considered a bad actor and a proposal is created and approved by 0x01 and 0x02 during block 2 but not yet executed to remove 0x03 from the member list and reduce minApprovals to 1.

0x03 can now swoop in with a MEV sandwich block 3 and execute the proposal and create a new proposal in the same block and auto-execute it resulting in 0x03 taking over the Multisig.

This problem is because createProposal() uses the members list from the previous block (2) but the new settings from block 3 resulting in 0x03 still being a member of the Multisig but having already minApprovals set to 1. So 0x03 can create any proposal, approve it and execute it in the same TX.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran all tests with success and extended them if necessary.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the test network.

[Rekard0]

LGTM!