Revert upgrade to HTMLUnit 2.23

This commit reverts the upgrade from HTMLUnit 2.11 to 2.23. As of HTMLUnit
2.22, `websocket-client` is no longer provided and triggers the following
issue:

SeleniumHQ/htmlunit-driver#22

See spring-projectsgh-7232
See spring-projectsgh-7895

Author: snicoll

spring-boot-dependencies/pom.xml

@@ -91,7 +91,7 @@
 		<hikaricp.version>2.5.1</hikaricp.version>
 		<hikaricp-java6.version>2.3.13</hikaricp-java6.version>
 		<hsqldb.version>2.3.3</hsqldb.version>
-		<htmlunit.version>2.23</htmlunit.version>
+		<htmlunit.version>2.21</htmlunit.version>
 		<httpasyncclient.version>4.1.2</httpasyncclient.version>
 		<httpclient.version>4.5.2</httpclient.version>
 		<httpcore.version>4.4.5</httpcore.version>
@@ -143,7 +143,7 @@
 		<reactor.version>2.0.8.RELEASE</reactor.version>
 		<reactor-spring.version>2.0.7.RELEASE</reactor-spring.version>
 		<selenium.version>2.53.1</selenium.version>
-		<selenium-htmlunit.version>2.23.2</selenium-htmlunit.version>
+		<selenium-htmlunit.version>2.21</selenium-htmlunit.version>
 		<sendgrid.version>2.2.2</sendgrid.version>
 		<servlet-api.version>3.1.0</servlet-api.version>
 		<simple-json.version>1.1.1</simple-json.version>