expat: bump to 2.7.4 on macOS 26

[p-linnane]

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SDK has been updated, so we need to bump this.

[Copilot]

Pull request overview

Updates the macOS 26 SDK pkg-config metadata for expat to match the SDK-provided version.

Changes:

• Bumps expat.pc from version 2.7.3 to 2.7.4.

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[MikeMcQuaid]

Thanks!

[MikeMcQuaid]

@p-linnane CI failing so we may need to wait until actions images updated or change how we test this (e.g. we look for this version but allow higher patch declared patch versions or something)

[p-linnane]

@MikeMcQuaid I opened this PR because we had CI failing complaining it was finding 2.7.3 when it expected 2.7.4. Looks like GitHub is in the middle of a deploy, so we could have hit an outdated runner on one job, and an updated one on another. Deployment started on the 11th, so should almost be done.

[p-linnane]

The job in merge queue used the previous deployment still: Version: 20260427.0026.1