Description about Security Center should be changed to Defender for Cloud

Azure Security Center has been renamed to Microsoft Defender for Cloud. Therefore, the description of Security Center(ex. category, description in [plugins/azure/securitycenter](https://github.com/aquasecurity/cloudsploit/tree/master/plugins/azure/securitycenter)) should be changed to Defender for Cloud.

[Microsoft Defender for Cloud - CSPM & CWPP | Microsoft Azure](https://azure.microsoft.com/en-us/services/defender-for-cloud/)

Start in plugins/azure/securitycenter and inspect the category and description text for the old Azure Security Center name. Update the referenced descriptions to Microsoft Defender for Cloud, then verify that no outdated naming remains in that plugin.

Written by the indexing model from the issue text.