Remove code in preparation for v5.0

[Amxx]

Fixes #3918

Fixes LIB-824
Fixes LIB-825
Fixes LIB-826

This PR removes:

• Escrows & PullPayment
• ERC777 & ERC1820Implementer
• CrossChain (+vendor)
• GovernorProposalThreshold
• Timers library
• Checkpoints.History
• ERC20.safeApprove
• AccessControl._setupRole
• ifAdmin modifier
• getters in the proxies
• getter with error string (in Math.sol and EnumerableMap.sol
• deprecated draft-xxx files
• PullPayment

Note:
We keep the IERC777 interfaces in the contracts/interfaces folder. Should plan a migration script that rewrite the import part for these ?

PR Checklist

• Tests
• Documentation
• Changeset entry (run npx changeset add)

[changeset-bot]

🦋 Changeset detected

Latest commit: 41732d3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
openzeppelin-solidity	Major

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[frangio]

Should plan a migration script that rewrite the import part for these ?

I wouldn't have a migration script this time because I expect that it will be very few renamed/moved files, we should list it in CHANGELOG.md under "How to upgrade from 4.x".

I would consider merging all the removal changesets into one, what do you think?

[frangio]

We need to explain the rationale for some of these removals, I think particularly this one, perhaps both in the PR description and the changesets/changelog.

[CodeSandwich]

So what's the rationale? There's none in the PR description and the changelog only states the removal 😆 Are these bridges or the removed contracts unsafe? Did universal bridges become so good that there's no reason to support custom ones?

[ernestognw]

The rationale is basically that the contracts had really low usage and the Crosschain landscape went in a different direction, this includes other bridges' technology and use cases.

I think the changeset is fine as it is right now, perhaps some of these notes may be added to the changelog in a different fashion other than a changeset entry.

[frangio]

We should remove Checkpoint.History as well (#4019). We had a PR for this that we may want to consider: #4170

[Amxx]

Removing History has been on my mind, but it was not listed anywhere, and we did not discuss it, so I wanted to bring it up latter. But if you agree, lets do it!

@ernestognw, is that ok with you as well ?

We should remove Checkpoint.History as well (#4019). We had a PR for this that we may want to consider: #4170

The PR is a bit old, an might create some conflicts. Possibly easier to re-do it here independently.

[RenanSouza2]

Removing History has been on my mind, but it was not listed anywhere, and we did not discuss it, so I wanted to bring it up latter. But if you agree, lets do it!

@ernestognw, is that ok with you as well ?

We should remove Checkpoint.History as well (#4019). We had a PR for this that we may want to consider: #4170

The PR is a bit old, an might create some conflicts. Possibly easier to re-do it here independently.

I made this new branch but did not open a new PR

https://github.com/RenanSouza2/openzeppelin-contracts/tree/next-v5.0-Checkpoint-history-v2

[RenanSouza2]

The PR is a bit old, an might create some conflicts. Possibly easier to re-do it here independently.

I closed that one myself for this reason and I also didn't want to spam PRs

[Amxx]

I would consider merging all the removal changesets into one, what do you think?

What syntax would you use? I think we should have one line per "important" removal.