[api-minor] Update the minimum supported Safari version to 16.4

This patch updates the minimum supported browsers as follows:
 - Safari 16.4, which was released on 2023-03-27; see https://developer.apple.com/documentation/safari-release-notes/safari-16_4-release-notes

Nowadays we usually we try, where feasible and possible, to support browsers/environments that are about two years old. The reasons for limiting support to a slightly more recent Safari version include:
 - Safari has always been slower, compared to other browsers, at implementing e.g. new JavaScript features.
 - Trying to provide support for Safari is often difficult, and over the years we have seen *a lot* of bugs that are specific to Safari.
 - Safari is, and has been for many years, only listed as "mostly" supported in the FAQ.
 - This allows us to remove feature-testing, only relevant to Safari, from the main code-base.

By limiting support to only "recent" browsers we reduce the risk of holding back improvements of the built-in Firefox PDF Viewer, and also (significantly) reduce the maintenance/support burden for the PDF.js core contributors.

*Please note:* As always, the minimum supported browser version assumes that a `legacy`-build of the PDF.js library is being used; see https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#faq-support

gulpfile.mjs

@@ -84,7 +84,7 @@ const ENV_TARGETS = [
   "last 2 versions",
   "Chrome >= 98",
   "Firefox ESR",
-  "Safari >= 15.4",
+  "Safari >= 16.4",
   "Node >= 18",
   "> 1%",
   "not IE > 0",