fix(build): declare the extensions.xml namespace so Renovate tracks Nisse

Renovate parses `.mvn/extensions.xml` by default, but its maven manager rejects
the file outright unless the root element carries a recognised EXTENSIONS
namespace -- parseExtensions() returns null when `attr.xmlns` is not one of
EXTENSIONS/1.0.0, 1.1.0 or 1.2.0, and the file is then skipped with no warning.

Ours declared no xmlns at all, so Nisse would have sat at 0.9.5 indefinitely
while every other dependency in the repo kept moving. Silent, and exactly the
kind of rot the pin was supposed to avoid.

Verified that Maven still resolves the version with the namespace present:
tagging 9.9.9 and running `mvn help:evaluate -Dexpression=project.version`
prints 9.9.9, unchanged.

Signed-off-by: Jimisola Laursen <jimisola@jimisola.com>

Author: jimisola

.mvn/extensions.xml

@@ -1,5 +1,10 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<extensions>
+<!-- The xmlns is load-bearing: Renovate's maven manager skips an extensions.xml
+     without a recognised EXTENSIONS namespace, so without it Nisse is never
+     updated. -->
+<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.0.0"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.0.0 https://maven.apache.org/xsd/core-extensions-1.0.0.xsd">
     <!-- Derives ${nisse.jgit.dynamicVersion} from git state, so the tag is the
          only version. See RELEASING.md in reqstool/.github. -->
     <extension>