Fix for testing plugins with older stable releases

[Haselnussbomber]

When a plugins last stable release is older than the last Dalamud API version, it wouldn't be shown in the Plugin installer, even if there is a testing version available for the current Dalamud API version.
This is currently happening for Quest Tracker (manifest), which has a DalamudApiLevel of 11, but a TestingDalamudApiLevel of 13.

There are two issues that come into play:

1. When searching for a plugin in the Plugin Installer, the function PluginInstallerWindow.IsManifestFiltered would filter out plugins that didn't match the current API level. This is redundant in my opinion, since the API version is checked in PluginManager.IsManifestEligible when the plugin repository loads.
2. The PluginManager.IsManifestEligible function on the other hand only checked the TestingDalamudApiLevel if the user already has opted in to testing of this plugin. I changed it so that the specific plugin testing opt-in state is not taken into consideration.

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For convenience, I moved RemotePluginManifest.IsAvailableForTesting up to IPluginManifest, implemented by the base class PluginManifest. With that, the function PluginManager.HasTestingVersion has been removed.