Add the 'supportsQt6' property in the plugins.xml, so it can be used to filter/gray out plugins

[rduivenvoorde]

A plugin author can now add 'supportsQt6' to make it clear that the plugin is (also) usable in a Qt6 build.

This propery is not yet availabe in the xml output. Eg https://plugins.qgis.org/plugins/plugins.xml?qgis=3.34

...
<pyqgis_plugin name="Delta Shares" version="0.3" plugin_id="3319">
        <description><![CDATA[With this plugin you can load data from delta shares]]></description>
        <about><![CDATA[This plugin enables you to load data directly from delta sharing implementaions, such as Databricks.]]></about>
        <version>0.3</version>
        <trusted>False</trusted>
        <qgis_minimum_version>3.0.0</qgis_minimum_version>
        <qgis_maximum_version>3.99.0</qgis_maximum_version>
        <homepage><![CDATA[https://github.com/Bartman0/q-delta-lake]]></homepage>
        <file_name>delta_lake.0.3.zip</file_name>
        <icon>/media/packages/2024/icon_kknQneG.png</icon>
        <author_name><![CDATA[Richard Kooijman]]></author_name>
        <download_url>https://plugins.qgis.org/plugins/delta_lake/version/0.3/download/</download_url>
        <uploaded_by><![CDATA[bartman0]]></uploaded_by>
        <create_date>2024-03-22T13:06:51.645792+00:00</create_date>
        <update_date>2024-03-22T13:06:53.082314+00:00</update_date>
        <experimental>True</experimental>
        <deprecated>False</deprecated>
        <tracker><![CDATA[https://github.com/Bartman0/q-delta-lake/issues]]></tracker>
        <repository><![CDATA[https://github.com/Bartman0/q-delta-lake]]></repository>
        <tags><![CDATA[database]]></tags>
        <downloads>5</downloads>
        <average_vote>0.0</average_vote>
        <rating_votes>0</rating_votes>
        <external_dependencies></external_dependencies>
        <server>False</server>
    </pyqgis_plugin>
...

Having this property would make it possible to, for example in QGIS from a Qt6 build, to gray out all plugins which are not ported to Qt6 yet. (hoping that that will encourage plugin authors or users to start porting)?