CocoaRestClient: bump min Darwin to 18 (10.14)

[nilFinx]

[skip ci]

Description

This port was updated to 1.(forgot...).7, but from .6, the app requires 10.14. This PR bumps the min version to the appropriate version, from 16 (10.12).

Type(s)

• bugfix
• enhancement
• security fix

Tested on

Unfortunately nowhere. But it should work in theory, this just changes a single version number.

Verification

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• tried existing tests with sudo port test?
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• checked that the Portfile's most important variants haven't been broken?