Deprecate/disable/remove for major/minor release

[MikeMcQuaid]

Prepare for the next major/minor release with the usual deprecation cycle.

See https://docs.brew.sh/Deprecating-Disabling-and-Removing

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• Have you followed our Contributing guidelines?
• Have you checked for other open Pull Requests for the same change?
• Have you explained what your changes do? Performance claims (e.g. "this is faster") must include brew benchmark results.
• Have you explained why you'd like these changes included, not just what they do?
• For bug fixes, have you given step-by-step brew commands to reproduce the bug?
• Have you written new tests (excluding integration tests)? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) locally?

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Currently done by hand, will shortly be using Codex 5.6 Sol xhigh and/or Claude Fable 5 high with local review and testing to address CI failures and other fixes.

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[samford]

There's some logic in livecheck that I've been meaning to deprecate (namely, the user agent fallback when a request fails, which was only used as a temporary measure until we could set the user_agent in a livecheck block like we can now). Would it be preferable to push a commit here to add odeprecated calls or handle it in a separate PR? [I'm fine with a separate PR but didn't want to add noise if the former is preferred.]