Install beta versions of the xmlrpc extension (#683)

There's no stable version published on PECL

MAINTAINERS.md

@@ -127,6 +127,11 @@ We should switch to the stable release once it will be available.
 The `xdiff` PHP extension uses the LibXDiff library.
 We need to monitor new releases at http://www.xmailserver.org/xdiff-lib.html
 
+### xmlrpc
+
+The only available versions of the `xmlrpc` PHP extension are all beta.
+We should switch to the stable release once it will be available.
+
 ### zookeeper
 
 The latest stable release of the `zookeeper` PHP extension doesn't support PHP 7.3+, so we install the alpha version.

install-php-extensions

@@ -3469,6 +3469,11 @@ installRemoteModule() {
 				addConfigureOption enable-reader yes
 			fi
 			;;
+		xmlrpc)
+			if test -z "$installRemoteModule_version"; then
+				installRemoteModule_version=beta
+			fi
+			;;
 		yac)
 			if test -z "$installRemoteModule_version"; then
 				if test $PHP_MAJMIN_VERSION -le 506; then