Proposed new policy: breaking changes get batched for annual cycle

[jamesliu4c]

This is in response to this issue (which, upon reflection, should probably have been a discussion instead). It only addresses one of the points there, but I think it is discrete enough that we can have a productive discussion about it.

I propose that setuptools have a release cadence for breaking changes, where they are only made under the following conditions:

1. In an annual major version release
2. In response to critical (or maybe critical and high) CVEs.

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From March 2024 to March 2025, we went from version 69.2.0 to 78.1.0, which is approximately a major version every month. This cadence makes it difficult to choose a major version to pin. If there isn't something like an annual cadence, I think the action people will take to avoid the next set of removals will be to pin to 78.0.2, which is also less than idea.

If the decision is made to adopt an annual major release cadence, this discussion should be considered complete when the month is chosen for that release.