Add baseline for indirect deprecations

We cannot fix these PHP deprecations, as these are happening in third
party code (and fixing them requires a major version bump of Flysystem).

PHPUnit 11 distinguishes direct and indirect deprecations, allowing us
to automatically ignore all indirect deprecations. However, this version
requires PHP 8.2, while this library still supports PHP 8.1.

phpunit-baseline.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0"?>
+<files version="1">
+ <file path="vendor/league/flysystem/src/FilesystemInterface.php">
+  <line number="274" hash="974a66e314c847f29d4b178ad6b47395be68d98f">
+   <issue><![CDATA[League\Flysystem\FilesystemInterface::get(): Implicitly marking parameter $handler as nullable is deprecated, the explicit nullable type must be used instead]]></issue>
+  </line>
+ </file>
+ <file path="vendor/league/flysystem/src/Filesystem.php">
+  <line number="363" hash="813ac25441e5e8e2323613ee4eee44ff5927e423">
+   <issue><![CDATA[League\Flysystem\Filesystem::get(): Implicitly marking parameter $handler as nullable is deprecated, the explicit nullable type must be used instead]]></issue>
+  </line>
+ </file>
+</files>

phpunit.xml.dist

@@ -30,7 +30,7 @@
         </testsuite>
     </testsuites>
     <coverage/>
-    <source ignoreSuppressionOfDeprecations="true">
+    <source ignoreSuppressionOfDeprecations="true" baseline="phpunit-baseline.xml">
         <include>
             <directory suffix=".php">packages/**/src</directory>
         </include>