Placeholder setups and Multisig Build 2 deployment

[heueristik]

Description

This PR adds a PlaceholderSetup that we can use to populate plugin repos with earlier versions to maintain the same release and build numbers on multiple chains/layers while saving gas.
Here, we use it to not upload build 1 of the MultisigSetup again when deploying to polygon.

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.

[mathewmeconry]

I would have chosen a different name than "dummy". Something like UndeployedVersion, VersionPlaceholder, or something when used throws a telling error to the user.