feat: remove code for unsupported version v5 of Guzzle

.changes/nextrelease/remove-guzzlev5-support.json

@@ -0,0 +1,7 @@
+[
+  {
+    "type": "enhancement",
+    "category": "Handler",
+    "description": "Remove code for unsupported version v5 of Guzzle"
+  }
+]

CONTRIBUTING.md

@@ -117,7 +117,7 @@ we ask the same of all community contributions as well:
 ### Changelog Documents
 
 A changelog document is a small JSON blob placed in the `.changes/nextrelease`
-folder. It should be named a clearly and uniquely, akin to a branch name. It
+folder. It should be named clearly and uniquely, akin to a branch name. It
 consists of a type, category, and description as follows:
 
 ```json

src/functions.php

@@ -279,11 +279,6 @@ function default_http_handler()
         return new \Aws\Handler\GuzzleV6\GuzzleHandler();
     }
 
-    // If Guzzle 5 installed
-    if ($version === 5) {
-        return new \Aws\Handler\GuzzleV5\GuzzleHandler();
-    }
-
     throw new \RuntimeException('Unknown Guzzle version: ' . $version);
 }
 
@@ -300,11 +295,6 @@ function default_user_agent()
         return \GuzzleHttp\default_user_agent();
     }
 
-    // If Guzzle 5 installed
-    if ($version === 5) {
-        return \GuzzleHttp\Client::getDefaultUserAgent();
-    }
-
     throw new \RuntimeException('Unknown Guzzle version: ' . $version);
 }
 
@@ -327,9 +317,6 @@ function guzzle_major_version()
         if ($version[0] === '6') {
             return $cache = 6;
         }
-        if ($version[0] === '5') {
-            return $cache = 5;
-        }
     } elseif (defined('\GuzzleHttp\ClientInterface::MAJOR_VERSION')) {
         return $cache = ClientInterface::MAJOR_VERSION;
     }