Fix Exports.swift

[gwynne]

This officially cleans up the last repo left that still has an uncorrected Exports.swift. Cleans up the README and CI workflows too.

[MahdiBM]

LGTM

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[gwynne]

Note: Even though I've bumped the minimum Swift version requirement here, which is normally a semver-minor change, I'm leaving this PR marked no-release-required, on account of 5.6 was already effectively required by the dependency on Vapor, meaning no actual change has taken place aside from silencing the complaints of the 5.4 SwiftPM not knowing about DocC catalogs.

[codecov-commenter]

Codecov Report

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Additional details and impacted files

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