Bump minimum SDK version to API 23

- Updated the `minSdk` in `app/build.gradle.kts`.
- Adjusted the api-level for the test-android min sdk CI workflow
  in `.github/workflows/ci.yml`.
- Simplified the `getOsString()` method in `ErrorActivity.java` by removing
  the conditional check for `Build.VERSION_CODES.M`, as API 23 is now the minimum.

.github/workflows/ci.yml

@@ -69,7 +69,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - api-level: 21
+          - api-level: 23
             target: default
             arch: x86
           - api-level: 35

app/build.gradle.kts

@@ -44,7 +44,7 @@ android {
     defaultConfig {
         applicationId = "org.schabi.newpipe"
         resValue("string", "app_name", "NewPipe")
-        minSdk = 21
+        minSdk = 23
         targetSdk = 35
 
         versionCode = System.getProperty("versionCodeOverride")?.toInt() ?: 1005

app/src/main/java/org/schabi/newpipe/error/ErrorActivity.java

@@ -307,8 +307,7 @@ public class ErrorActivity extends AppCompatActivity {
     }
 
     private String getOsString() {
-        final String osBase = Build.VERSION.SDK_INT >= Build.VERSION_CODES.M
-                ? Build.VERSION.BASE_OS : "Android";
+        final String osBase = Build.VERSION.BASE_OS;
         return System.getProperty("os.name")
                 + " " + (osBase.isEmpty() ? "Android" : osBase)
                 + " " + Build.VERSION.RELEASE