Add licenses by SPDX identifier, mark shorthand aliases as deprecated

[AndyScherzinger]

matching the app store change at nextcloud/appstore#1560

What puzzles me a bit is that the app store has:

<xs:enumeration value="agpl"/>
<xs:enumeration value="mit"/>
<xs:enumeration value="mpl"/>
<xs:enumeration value="apache"/>
<xs:enumeration value="gpl3"/>

While the server has less values:

<xs:enumeration value="agpl"/>
<xs:enumeration value="mpl"/>
<xs:enumeration value="apache"/>

while the app store list is fine in terms of license compatibility

• Resolves: More detailed licensing information appstore#1274

Summary

TODO

• get feedback / review on the change

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)