docs: update CI badges

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

README.md

@@ -1,7 +1,6 @@
 # Amazon S3 plugin for [Fluentd](http://github.com/fluent/fluentd)
 
-[<img src="https://travis-ci.org/fluent/fluent-plugin-s3.svg?branch=master"
-alt="Build Status" />](https://travis-ci.org/fluent/fluent-plugin-s3) [<img
+[![linux](https://github.com/fluent/fluent-plugin-s3/actions/workflows/linux.yml/badge.svg)](https://github.com/fluent/fluent-plugin-s3/actions/workflows/linux.yml) [<img
 src="https://codeclimate.com/github/fluent/fluent-plugin-s3/badges/gpa.svg"
 />](https://codeclimate.com/github/fluent/fluent-plugin-s3)