Update brew cask documentation

The command `brew cask` has been deprecated:
<https://github.com/Homebrew/discussions/discussions/340#discussioncomment-232364>

Should be `brew install` directly now.

PR #2231 <https://github.com/Genymobile/scrcpy/pull/2231>

Signed-off-by: Romain Vimont <rom@rom1v.com>

BUILD.md

@@ -208,7 +208,7 @@ make it avaliable from the `PATH`:
 
 ```bash
 brew tap homebrew/cask-versions
-brew cask install adoptopenjdk/openjdk/adoptopenjdk8
+brew install adoptopenjdk/openjdk/adoptopenjdk8
 export JAVA_HOME="$(/usr/libexec/java_home --version 1.8)"
 export PATH="$JAVA_HOME/bin:$PATH"
 ```

README.md

@@ -127,11 +127,7 @@ brew install scrcpy
 You need `adb`, accessible from your `PATH`. If you don't have it yet:
 
 ```bash
-# Homebrew >= 2.6.0
-brew install --cask android-platform-tools
-
-# Homebrew < 2.6.0
-brew cask install android-platform-tools
+brew install android-platform-tools
 ```
 
 You can also [build the app manually][BUILD].