Remove deps, and modify contracts to support the change

[MilGard91]

Context

Issue: #929

Changes proposed in this pull request

• Remove solmate and @OpenZepelin v2 contracts
  Note: instead of copying the SafeMath.sol to the external directory in the governance, copied it to lib/external since other contracts use that are not in governance directory
  *Note: leaving @openzeppelin/contracts-v4 as it is, if we rename it without the -v4 it breaks the beacon proxy and it's bytecode so the StakingPool addresses are calculated wrong *

Test plan

• N/A

Checklist

• Rebased the base branch
• Attached corresponding Github issue
• Prefixed the name with the type of change (i.e. feat, chore, test)
• Performed a self-review of my own code
• Followed the style guidelines of this project
• Made corresponding changes to the documentation
• Didn't generate new warnings
• Didn't generate failures on existing tests
• Added tests that prove my fix is effective or that my feature works

Review

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