[browser docs] Update "supported browsers" page

[lobsterkatie]

Now that the JS SDK is ES6-first, our "Supported Browsers" page is out of date.

Tasks:

• Figure out minimum supported version of each browser, and mark that rather than just saying "latest."
• Note where lower browser versions can be supported by using an ES5 bundle or down-compiling the code, linking to https://docs.sentry.io/platforms/javascript/install/cdn/#available-bundles and https://docs.sentry.io/platforms/javascript/troubleshooting/#supporting-older-browsers.
• Decide if it's worth it to keep the stuff about using IE 10 with SDK versions below 5.7.0.

[getsentry-release]

Routing to @getsentry/team-web-sdk-frontend for triage. ⏲️

[github-actions]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

---

"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[timfish]

Figure out minimum supported version of each browser

Merging this globalThis simplification PR would take the minimum supported versions a bit beyond ES6 to:

Browser	Version
Chrome	71
Edge	79
Firefox	65
Safari	12.1
Safari iOS	12.2

From Android 5.0, the WebView changed to using an updatable APK which when auto-updated keeps step with Chrome so I'm not sure we need to consider Android separately any more?