chore(deps): bump cos_api from 5.6.18 to 5.6.21 (#794)

Bumps [cos_api](https://github.com/tencentyun/cos-java-sdk-v5) from 5.6.18 to 5.6.21.
- [Release notes](https://github.com/tencentyun/cos-java-sdk-v5/releases)
- [Changelog](https://github.com/tencentyun/cos-java-sdk-v5/blob/master/CHANGELOG.md)
- [Commits](https://github.com/tencentyun/cos-java-sdk-v5/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

build.gradle

@@ -57,7 +57,7 @@ ext {
     qiniuSdkVersion =  "7.2.28"
     aliyunSdkVersion =  "3.8.1"
     baiduSdkVersion =  "0.10.36"
-    qcloudSdkVersion =  "5.6.18"
+    qcloudSdkVersion =  "5.6.21"
     swaggerVersion =  "2.9.2"
     commonsLangVersion =  "3.9"
     httpclientVersion =  "4.5.12"