GH-46002: [CI][Release] Define an official Arrow C++ support policy

[raulcd]

Warning

This PR is under discussion and active development.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

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This PR includes breaking changes to public APIs. (If there are any breaking changes to public APIs, please explain which changes are breaking. If not, you can remove this.)

This PR contains a "Critical Fix". (If the changes fix either (a) a security vulnerability, (b) a bug that caused incorrect or invalid data to be produced, or (c) a bug that causes a crash (even when the API contract is upheld), please provide explanation. If not, you can remove this.)

• GitHub Issue: [C++] Define an official support policy #46002

[github-actions]

⚠️ GitHub issue #46002 has been automatically assigned in GitHub to PR creator.

[pitrou]

Thanks for starting this! A general question first: should this be a project-wide document? Should this three-tier approach apply to all implementations?

[raulcd]

That's a good question. I suppose this could potentially apply to several implementations but some specifics, i.e how to identify tier 2 jobs, won't necessary have to be the same between the different sub-projects / implementations.
I'll move the document under C++ implementation and focus on the C++ implementation specifically. We can include other implementations and iterate over it later on.

[amoeba]

Thanks @raulcd. This is looking about right. Can we also include a section for change history? We'd document changes (additions, removals, promotions, demotions) there.

I also wonder how we feel about platforms with what we might call "externally supported", like #46002 (comment) and whether we might allow platforms to stay in tier1/2 despite being EOL'd upstream. For these platforms, we'd want to document who is pledging support and how to contact them to fix issues.