Update plugin structure requirements for upload to directory

[ioalexei]

Fixes #9192 :

• add reference to required LICENSE file

Goal: clarify required files for validation in the official plugin repository

Ticket(s): #9192

• Backport to LTR documentation is requested

[DelazJ]

@Xpirix may I request your feedback here please? Thanks.

[Xpirix]

Thanks for this change @ioalexei and @DelazJ. This reflects the current plugin upload requirements and will likely help authors successfully upload their plugins. Is it possible to mention that the requirement is valid for both new plugins and new versions of existing plugins?

In the future, the LICENSE requirement may change depending on the discussion at qgis/QGIS-Django#453, but I think this looks good for now.

[Gustry]

Thanks @ioalexei
It's not the topic in this PR, but I would also remove the resources file. It's mostly not used anymore and I do not recommend it (and also L136, 137)

[ioalexei]

• Merged suggestions from @DelazJ and @Gustry
• Fixed a typo
• Added clarification that it applies to updates as well as newly published plugins
• Mentioned the license requirement in the plugin structure setting as well