Append removal of old button variables to the migration guide

[schaertim]

This PR #4702 removed/refactored some variables not used for the post-button anymore. They should be mentioned in the migration guide.
Here is a list of them:

Refactored:

• $input-btn-border-width
• $input-btn-padding-y
• $input-btn-padding-x
• $input-btn-padding-y-sm
• $input-btn-padding-x-sm
• $input-btn-padding-y-rg
• $input-btn-padding-x-rg
• $input-btn-padding-y-lg
• $input-btn-padding-x-lg
• $input-btn-focus-width

Completely Removed:

• $btn-height-sm
• $btn-height-md
• $btn-disabled-opacity
• $btn-focus-width
• $btn-focus-box-shadow
• $btn-icon-size-sm
• $btn-icon-size-md
• $btn-icon-size-rg
• $btn-icon-size-lg
• $input-btn-border-width-rem
• $input-btn-line-height
• $input-btn-focus-color
• $input-btn-focus-box-shadow
• $old-btn-icon-size-map