enh: set ooxml as default doc_format

[grnd-alt]

• Resolves: Switch to OOXML for improved interoperability #4812

[juliusknorr]

There seem to be a few more occurences of the doc_format config key that I think also need an adjustment:

lib/Settings/Admin.php
48:					'doc_format' => $this->config->getAppValue('richdocuments', 'doc_format'),

lib/TemplateManager.php
416:		$ooxml = $this->config->getAppValue(Application::APPNAME, 'doc_format', '') === 'ooxml';
445:		$ooxml = $this->config->getAppValue(Application::APPNAME, 'doc_format', '') === 'ooxml';

lib/Service/SlideDeckService.php
44:		$ooxml = $this->config->getAppValue(Application::APPNAME, 'doc_format', '') === 'ooxml';

[juliusknorr]

Also please add relevant labels