Add support for more complex button backgrounds

[DiegoBM-DB]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

At the moment the mixin for the button component background only supports background-color, it would be desirable to support more complex backgrounds like gradients, etc

What does the proposed API look like?

Replacing "background-color: @background;" with "background: @background;", that will still support background color as it is, so it would not be a breaking change, and at the same time it would allow for more complex backgrounds like gradients

[ant-design-bot]

Hello @DiegoBM-DB. We totally like your proposal/feedback, welcome to send us a Pull Request for it. Please send your Pull Request to proper branch (feature branch for the new feature, master for bugfix and other changes), fill the Pull Request Template here, provide changelog/TypeScript/documentation/test cases if needed and make sure CI passed, we will review it soon. Appreciate it advance and we are looking forward to your contribution!

你好 @DiegoBM-DB, 我们完全同意你的提议/反馈，欢迎直接在此仓库 创建一个 Pull Request 来解决这个问题。请将 Pull Request 发到正确的分支（新特性发到 feature 分支，其他发到 master 分支），务必填写 Pull Request 内的预设模板，提供改动所需相应的 changelog、TypeScript 定义、测试用例、文档等，并确保 CI 通过，我们会尽快进行 Review，提前感谢和期待您的贡献！