RFR: 8369226: GHA: Switch to MacOS 15

[Erik Joelsson]

On Mon, 6 Oct 2025 17:34:45 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Current GHA workflows for MacOS are running with:
>  - x86_64: macos-13, builds only
>  - aarch64: macos-14, builds+tests
> 
> GitHub is deprecating MacOS 13 runner on December 4th, 2025, the brownouts are starting in a month: https://github.com/actions/runner-images/issues/13046. Additionally, GHA phases out MacOS x86_64, and MacOS 15 would be the last runner for x86_64. We have already disabled testing jobs for MacOS x86_64. GitHub recommends migrating to MacOS 15 at this time.
> 
> We can switch to MacOS 15 runner for both x86 and AArch64 to keep pipelines green in the coming months.
> 
> Additional testing:
>  - [ ] GHA

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27655#pullrequestreview-3306547175