Update to a newer version of the Swift-DocC Plugin

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This only updates the resolved version of the swift-docc-plugin dependency to 1.4+ so that both DocC and the plugin depend on the "github.com/swiftlang/swift-docc-symbolkit" link to SymbolKit rather than the "github.com/apple/swift-docc-symbolkit" link.

This fixes a warning when building DocC:

Conflicting identity for swift-docc-symbolkit: dependency 'github.com/apple/swift-docc-symbolkit' and dependency 'github.com/swiftlang/swift-docc-symbolkit' both point to the same package identity 'swift-docc-symbolkit'.

Because the 1.3 dependency was previously specified in the Package.resolved file. Anyone who cloned DocC and built it would encounter this warning.

Dependencies

Update to a newer version of the swift-docc-plugin dependency.

Testing

Nothing in particular. This isn't a user facing change.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [ ] Added tests
• Ran the ./bin/test script and it succeeded
• [ ] Updated documentation if necessary

[d-ronnqvist]

We might consider holding this PR until swiftlang/swift-docc-plugin#107 is merged and released as a new tag so that we can pick up those resolve those warnings at the same time.

[d-ronnqvist]

@swift-ci please test