Allow tier badges to use round brackets

This MR allows product badge tiers to be either **[ ]** or **( )**.

This will allow us to transition tier badges away from markup that causes Kramdown warnings to a markup that won't cause those warnings.

Once all tiers badges are converted, we could remove the ability to use **[ ]** style.

@axil Looked good when I tested locally. Please also test before you merge.

@gl-docsteam

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63800
Related to https://gitlab.com/gitlab-com/gitlab-docs/issues/291.