[Bug] Fix GetEventCoverImageUrl by removing the GuildId in the Url string

[Twinki14]

See #2576 for more information

GuildScheduledEvent cover image Urls don't include the related {guildId} in the Url, GuildScheduledEvent cover image Urls are stored on Discord and fetched by only their {eventId} and {assetId}

Invalid Uri: guild-events/1068697076323602503/1069337361063878727/516b0a9fe85c696678d98750589aef5e.png?size=1024
Valid Uri: guild-events/1069337361063878727/516b0a9fe85c696678d98750589aef5e.png?size=1024

Invalid Url: https://cdn.discordapp.com/guild-events/1068697076323602503/1069337361063878727/516b0a9fe85c696678d98750589aef5e.png?size=1024
Valid Url: https://cdn.discordapp.com/guild-events/1069337361063878727/516b0a9fe85c696678d98750589aef5e.png?size=1024

A temporary fix can be achieved by using Replace or any other string manipulation technique

 var coverImageUrl = message.CoverImageId != null ? message.GetCoverImageUrl().Replace($"/{message.Guild.Id}", "") : null;

This closes #2576