Update extensions NDK version to latest LTS version

Latest LTS version is better than latest stable version because it will
be supported for longer.

#minor-release
Issue:#8581
PiperOrigin-RevId: 359467482

RELEASENOTES.md

@@ -24,13 +24,13 @@
 *   Text:
     *   Parse SSA/ASS font size in `Style:` lines
         ([#8435](https://github.com/google/ExoPlayer/issues/8435)).
-*   VP9 extension: Update to use NDK r22
+*   VP9 extension: Update to use NDK r21
     ([#8581](https://github.com/google/ExoPlayer/issues/8581)).
-*   FLAC extension: Update to use NDK r22
+*   FLAC extension: Update to use NDK r21
     ([#8581](https://github.com/google/ExoPlayer/issues/8581)).
-*   Opus extension: Update to use NDK r22
+*   Opus extension: Update to use NDK r21
     ([#8581](https://github.com/google/ExoPlayer/issues/8581)).
-*   FFmpeg extension: Update to use NDK r22
+*   FFmpeg extension: Update to use NDK r21
     ([#8581](https://github.com/google/ExoPlayer/issues/8581)).
 
 ### 2.13.1 (2021-02-12)

extensions/ffmpeg/README.md

@@ -30,7 +30,7 @@ FFMPEG_EXT_PATH="${EXOPLAYER_ROOT}/extensions/ffmpeg/src/main"
 ```
 
 * Download the [Android NDK][] and set its location in a shell variable.
-  This build configuration has been tested on NDK r22.
+  This build configuration has been tested on NDK r21.
 
 ```
 NDK_PATH="<path to Android NDK>"

extensions/flac/README.md

@@ -29,7 +29,7 @@ FLAC_EXT_PATH="${EXOPLAYER_ROOT}/extensions/flac/src/main"
 ```
 
 * Download the [Android NDK][] and set its location in an environment variable.
-  This build configuration has been tested on NDK r22.
+  This build configuration has been tested on NDK r21.
 
 ```
 NDK_PATH="<path to Android NDK>"

extensions/opus/README.md

@@ -29,7 +29,7 @@ OPUS_EXT_PATH="${EXOPLAYER_ROOT}/extensions/opus/src/main"
 ```
 
 * Download the [Android NDK][] and set its location in an environment variable.
-  This build configuration has been tested on NDK r22.
+  This build configuration has been tested on NDK r21.
 
 ```
 NDK_PATH="<path to Android NDK>"

extensions/vp9/README.md

@@ -29,7 +29,7 @@ VP9_EXT_PATH="${EXOPLAYER_ROOT}/extensions/vp9/src/main"
 ```
 
 * Download the [Android NDK][] and set its location in an environment variable.
-  This build configuration has been tested on NDK r22.
+  This build configuration has been tested on NDK r21.
 
 ```
 NDK_PATH="<path to Android NDK>"