Make instructions re: non-notarized PR tester build more user friendly

For security reasons, the macOS tester builds generated for PRs from forks are not notarized. Instructions are provided for
beta testing under these conditions.

Previously, the instructions for bypassing the macOS notarization requirement involved disabling macOS Gatekeeper
entirely using the spctl command. These instructions are hereby replaced by an alternative approach, where the
restriction is bypassed for the Arduino IDE application alone, via the macOS GUI.

The new approach is superior for the following reasons:

- More secure due to having limited scope
- More accessible due to the use of the macOS GUI

docs/contributor-guide/beta-testing.md

@@ -65,9 +65,11 @@ Build the version of the Arduino IDE you want to test from source instead of usi
 
 ---
 
-1. Use [this guide](https://help.apple.com/xcode/mac/10.2/index.html?localePath=en.lproj#/dev9b7736b0e), in order to disable Gatekeeper (at your own risk!).
-1. Beta test the tester build.
-1. Re-enable Gatekeeper after tests are done, following the guide linked above.
+1. Start the tester build.<br />
+   A warning will appear:
+   > "Arduino IDE" cannot be opened because the developer cannot be verified.
+1. Follow the instructions from the "**If you want to open an app that hasn't been notarized or is from an unidentified developer**" section of this page to bypass the security restriction:<br />
+   [https://support.apple.com/en-us/HT202491](https://support.apple.com/en-us/HT202491#:~:text=If%20you%20want%20to%20open%20an%20app%20that%20hasn%E2%80%99t%20been%20notarized%20or%20is%20from%20an%20unidentified%20developer)
 
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