misc: Upgrade Go to v1.21

Upgrade to 1.21 as 1.20 is now unmaintained.

Most changes are related to the new `slices` package from standard
library, which can replace some existing custom functions.

.github/workflows/build-macos.yml

@@ -24,7 +24,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: '1.20'
+        go-version: '1.21'
 
     - name: Install Dependencies
       run: brew install --force mpv && brew install dylibbundler
@@ -60,4 +60,4 @@ jobs:
       with:
         name: Supersonic_mac_x64_HighSierra+.zip
         path: Supersonic_HighSierra+.zip
-      
+

.github/workflows/build-ubuntu.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: '1.20'
+        go-version: '1.21'
 
     - name: Install dependencies
       run: sudo apt update && sudo apt install libmpv-dev gcc libegl1-mesa-dev xorg-dev

.github/workflows/build-windows.yml

@@ -29,7 +29,7 @@ jobs:
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: '1.20'
+        go-version: '1.21'
 
     - name: Install Dependencies
       run: >