tdf#158237 Use C++20 contains() instead of find() and end()

Change-Id: Id295063a5f0460ab0b66ced5be29aaaeceaa8382
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/182195
Tested-by: Jenkins
Reviewed-by: David Gilbert <freedesktop@treblig.org>

avmedia/source/gstreamer/gstplayer.cxx

@@ -138,7 +138,7 @@ void MissingPluginInstaller::report(
     rtl::Reference<MissingPluginInstallerThread> launch;
     {
         std::unique_lock g(mutex_);
-        if (reported_.find(detStr) != reported_.end()) {
+        if (reported_.contains(detStr)) {
             return;
         }
         auto & i = queued_[detStr];