change "litecoin" into a filled variant

[ondrejnedoma]

Before submitting a new icon contribution, please confirm the following:

• [X] I have searched all issues to make sure there isn't a request for this icon.
• [X] I have searched the current library to make sure the icon doesn't already exist.
• [X] I understand that brand icons are no longer being added and I can use SimpleIcons for my brand icon needs.
• [X] I have followed the Material guidelines for system icons.

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The "litecoin" icon exists, but is currently just a simple outline. Trying to match the bitcoin icon, I created this, which I think could replace the current litecoin. Closing #7330 by creating this issue (I thought this repo works on pull requests :D)

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[Templarian]

@ondrejnedoma That icon is deprecated and being removed in 8.0. All brand icons are being removed from the icon pack as Simple Icons is a much better alternative. See #6602 for more details on this decision.

https://pictogrammers.com/library/mdi/icon/litecoin/