FreeMarker integration is broken on Windows due to path issues

[vy]

As this failed CI run indicates:

java.lang.RuntimeException: failed exporting release from directory `D:\a\logging-log4j2\logging-log4j2\src\changelog\2.0-alpha1`
    at org.apache.logging.log4j.changelog.exporter.ChangelogExporter.main (ChangelogExporter.java:71)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:279)
    at java.lang.Thread.run (Thread.java:750)
Caused by: java.lang.RuntimeException: failed rendering template `D:\a\logging-log4j2\logging-log4j2\src\changelog\2.0-alpha1\.changelog.adoc.ftl` to file `D:\a\logging-log4j2\logging-log4j2\target\generated-sources\site\asciidoc\changelog\2.0-alpha1.adoc`
    at org.apache.logging.log4j.changelog.exporter.FreeMarkerUtils.render (FreeMarkerUtils.java:80)
    at org.apache.logging.log4j.changelog.exporter.ChangelogExporter.exportRelease (ChangelogExporter.java:170)
    at org.apache.logging.log4j.changelog.exporter.ChangelogExporter.exportRelease (ChangelogExporter.java:139)
    at org.apache.logging.log4j.changelog.exporter.ChangelogExporter.main (ChangelogExporter.java:63)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:279)
    at java.lang.Thread.run (Thread.java:750)
Caused by: freemarker.template.TemplateNotFoundException: Template not found for name "D:\\a\\logging-log4j2\\logging-log4j2\\src\\changelog\\2.0-alpha1\\.changelog.adoc.ftl".
The name was interpreted by this TemplateLoader: FileTemplateLoader(baseDir="D:\", canonicalBasePath="D:\").
Warning: The name contains backslash ("\") instead of slash ("/"); template names should use slash only.
    at freemarker.template.Configuration.getTemplate (Configuration.java:[28](https://github.com/apache/logging-log4j2/actions/runs/3943007132/jobs/6747337526#step:9:29)83)
    at freemarker.template.Configuration.getTemplate (Configuration.java:2685)
    at org.apache.logging.log4j.changelog.exporter.FreeMarkerUtils.render (FreeMarkerUtils.java:63)
    at org.apache.logging.log4j.changelog.exporter.ChangelogExporter.exportRelease (ChangelogExporter.java:170)
    at org.apache.logging.log4j.changelog.exporter.ChangelogExporter.exportRelease (ChangelogExporter.java:139)
    at org.apache.logging.log4j.changelog.exporter.ChangelogExporter.main (ChangelogExporter.java:63)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:279)
    at java.lang.Thread.run (Thread.java:750)

[vy]

Fixed in fd84623.