install-from-source: update minimum ruby version (#569)

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

installation/install-from-source.md

@@ -4,7 +4,7 @@ This article explains how to install Fluentd from source \(git repository\). Thi
 
 ## Step 1: Install Ruby Interpreter
 
-Install Ruby `>= 2.1` and `bundler` on your local environment.
+Install Ruby `>= 2.7` and `bundler` on your local environment.
 
 ## Step 2: Fetch Source Code