ci: run swift tests on macos-15

[reez]

Description

GitHub is deprecating Actions macOS 14 runners starting July 6, 2026, with full retirement on November 2, 2026, so this moves the Swift workflows to macOS 15.

actions/runner-images#13518

Notes to the reviewers

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other:

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added a changelog in the next release tracking issue (see example)
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR