Prioritize the experimental flag in the metadata

[Xpirix]

Fix for #477

In the Upload new plugins/version form, the Experimental checkbox says: Check this box if this version is experimental, leave unchecked if it's stable. Please note that this field might be overridden by metadata (if present). So it would make sense to prioritise the experimental attribute in the metadata and get the value from the checkbox if it's not set.

This change removes the extra validation that compares both values and allows the expected workflow.

[Gustry]

Side question, shouldn't we have a same behavior for the server checkbox ? Ref #487
It would make sense, if it's not already the case ?

[Xpirix]

@Gustry ( also replying to #487 (comment))

The primary distinction is that the server checkbox pertains to the plugin itself, while the experimental checkbox applies only to a specific version. Currently, the server checkbox takes precedence and can be activated either manually or by setting the server flag in the metadata of the initial plugin.

But yes, it would make sense to prioritise the value of the server field from the metadata.txt file whenever it is present. If we adopt that approach, a new version of the plugin will override the general information and settings, as we currently do for fields like Description, About, and others.

[Gustry]

Thanks for these clarifications. You can let it like this for now. If was mainly to keep notes.