Use httpclient 5.3.1, not httpclient 5.4

Reverts 1991154 from
jenkinsci#57

jenkinsci/docker-plugin#1032 describes the
failure that end users encounter with httpclient 5.4 when connecting to
the Docker daemon over a Unix domain socket.

jenkinsci/docker-plugin#1103 describes the
test automation failure that happens with httpclient 5.4.

jenkinsci#29
notes that there may have been a breaking change in httpclient 5.4.

jenkinsci#59
also notes that there is an issue in httpclient 5.4.

pom.xml

@@ -43,7 +43,7 @@
 
   <properties>
     <gitHubRepo>jenkinsci/apache-httpcomponents-client-5-api-plugin</gitHubRepo>
-    <httpclient.version>5.4</httpclient.version>
+    <httpclient.version>5.3.1</httpclient.version>
     <httpcore.version>5.3.1</httpcore.version>
     <jenkins.version>2.361.4</jenkins.version>
     <revision>${httpclient.version}</revision>