[Feature] update bootstrap

[wildskyf]

Thanks to @github 's alert, we noticed this issue.

Here are those deprecated things:

• img-retina
• invisible
• float
• size And neither of all are not used by us, nothing to do.

Related posts:

• https://blog.getbootstrap.com/2019/02/13/bootstrap-4-3-1-and-3-4-1/
• https://blog.getbootstrap.com/2019/02/11/bootstrap-4-3-0/
• https://blog.getbootstrap.com/2018/12/21/bootstrap-4-2-1/