Add usage of box shadow variable

[imanmalekian31]

Description

When I wanted to use popover box shadow, I noticed there is $popover-box-shadow but it is never used and doesn't get applied to popover. So I've added box-shadow to popover class.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[julien-deramond]

Hello @imanmalekian31
Thanks for creating a PR. However, as mentioned in our Contributing Guidelines:

Do not edit bootstrap.css or bootstrap.js, and do not commit any dist files (dist/ or js/dist). Those files are automatically generated by our build tools. You should edit the source files in /bootstrap/scss/ and/or /bootstrap/js/src/ instead.

Closing this PR. Feel free to open a new one.