Restore indentation in published spring-boot-dependencies pom

The move to an HTTPS URL for the xmlns:xslt identifier has the unwanted
side-effect of disabling indentation.

This commit moves back to an HTTPS URL. It also changes the indent size
to 2, aligning with the size used by all the other poms that are written
by the flatten plugin.

Closes gh-16466

spring-boot-project/spring-boot-dependencies/src/main/xslt/post-process-flattened-pom.xsl

@@ -3,7 +3,7 @@
 	xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:m="http://maven.apache.org/POM/4.0.0"
 	exclude-result-prefixes="m">
 	<xsl:output method="xml" encoding="utf-8" indent="yes"
-		xslt:indent-amount="4" xmlns:xslt="https://xml.apache.org/xslt" />
+		xslt:indent-amount="2" xmlns:xslt="http://xml.apache.org/xslt" />
 	<xsl:strip-space elements="*" />
 	<xsl:template match="@*|node()">
 		<xsl:copy>