[5.6] Set relatedDependenciesBranch to release/5.6

[MaxDesiatov]

Motivation:

Tools depending on release/5.6 branch of SwiftPM together with release/5.6 branch of TSC fail to build due to conflicting dependencies.

Modifications:

relatedDependenciesBranch now is set to release/5.6 instead of main when SwiftPM release/5.6 branch is checked out. This makes dependencies consistent and resolvable when other Swift tools are included in the dependency tree.

Result:

Tools depending on release/5.6 branch of SwiftPM can be built without failures.

[MaxDesiatov]

@swift-ci please test

[abertelrud]

Thanks @MaxDesiatov. Also opened a corresponding one on swift-driver since local builds will fail otherwise: swiftlang/swift-driver#929

[abertelrud]

Will mark approved once the swift-driver change is in so local desktop use doesn't break. Thanks for this fix @MaxDesiatov!

[abertelrud]

Still waiting for there to be a 5.6 toolchain so that we can merge the SwiftDriver change. Otherwise all non-CI builds of SwiftPM will start failing when we merge this.

[abertelrud]

Update: since there's no 5.6 toolchain yet, and therefore swiftlang/swift-driver#929 is blocked from merging, I ran a cross-repo test using an empty-commit PR in the Swift compiler repo (swiftlang/swift#40613). That seems to have succeeded, which means that we should be able to merge swiftlang/swift-driver#929 soon and then this one after that.

[abertelrud]

Update: Still trying to get the swift-driver change in, otherwise desktop builds of SwiftPM will start failing once this is merged.

[abertelrud]

The SwiftDriver change has been merged, so I will do a local test to confirm that this is now fixed, and then merge it.

[abertelrud]

With the latest change to SwiftDriver (swiftlang/swift-driver#964), this looks good now for local builds as well. I think we just need @tomerd's approval now for the 5.6 branch.