feat!: Remove deprecated features, update upgrade guides

[joeyparrish]

Below are the changelog entries for each deprecated feature removed by this commit.

---

feat(config)!: manifest.dash.defaultPresentationDelay has been replaced by manifest.defaultPresentationDelay (deprecated in v3.0.0)

feat(config)!: Configuration of factories should be plain factory functions, not constructors; these will not be invoked with new (deprecated in v3.1.0)

feat(player)!: shaka.Player.prototype.addTextTrack() has been replaced by addTextTrackAsync(), which returns a Promise (deprecated in v3.1.0)

feat(ui)!: shaka.ui.TrackLabelFormat has been renamed to shaka.ui.Overlay.TrackLabelFormat (deprecated in v3.1.0)

feat(ui)!: shaka.ui.FailReasonCode has been renamed to shaka.ui.Overlay.FailReasonCode (deprecated in v3.1.0)

feat(offline)!: shaka.offline.Storage.prototype.store() returns AbortableOperation instead of Promise (deprecated in v3.0.0)

feat(offline)!: shaka.offline.Storage.prototype.getStoreInProgress() has been removed; concurrent operations are supported, so callers don't need to check this (deprecated in v3.0.0)

feat!: shaka.util.Uint8ArrayUtils.equal has been replaced by shaka.util.BufferUtils.equal, which can handle multiple types of buffers (deprecated in v3.0.0)

feat(manifest)!: shaka.media.SegmentIndex.prototype.destroy() has been replaced by release(), which is synchronous (deprecated in v3.0.0)

feat(manifest)!: shaka.media.SegmentIterator.prototype.seek(), which mutates the iterator, has been replaced by shaka.media.SegmentIndex.getIteratorForTime() (deprecated in v3.1.0)

feat(manifest)!: shaka.media.SegmentIndex.prototype.merge() has become private; use mergeAndEvict() instead (deprecated in v3.2.0)

feat(plugin)!: AbrManager plugins must implement the playbackRateChanged() method (deprecated in v3.0.0)

feat(plugin)!: shaka.extern.Cue.prototype.spacer has been replaced by the more clearly-named lineBreak (deprecated in v3.1.0)

feat(plugin)!: IUIElement plugins must have a release() method (not destroy()) (deprecated in v3.0.0)

[joeyparrish]

Ping! @theodab, could you please look at this? Merging this wouldn't require us to instantly launch v4, but it would be nice to have this done and ready.

[theodab]

LGTM otherwise.