Update Safari version for TextTrackList's getTrackById()

It was implemented here:
WebKit/WebKit@b195c16
https://github.com/WebKit/WebKit/blob/b195c162a9805acacbb7d32328ba6fbbc1df4453/Source/WebCore/Configurations/Version.xcconfig

WebKit trunk version 538.8.0 maps to Safari 8 on both macOS and iOS.

Confirmed with tested in iOS 7 and 8:
mdn#16594 (comment)

This was last updated based on the collector here:
mdn#7929

But that was based on results from Safari 7.1:
foolip/mdn-bcd-results@5067a4d

api/TextTrackList.json

@@ -150,7 +150,7 @@
             "opera": "mirror",
             "opera_android": "mirror",
             "safari": {
-              "version_added": "7"
+              "version_added": "8"
             },
             "safari_ios": "mirror",
             "samsunginternet_android": "mirror",