Fix invalid margin-bottom value for node__meta class

Problem/Motivation

The value of margin-bottom for node__meta class is missing the unit.

Steps to reproduce

Given that Vartheme is enabled
When authoring information with node__meta class exists
Then it will have invalid margin-bottom value.

Deprecation Warning 
This operation is parsed as:

    0 - ($spacer)

but you may have intended it to mean:

    0 (-($spacer))

Add a space after - to clarify that it's meant to be a binary operation, or wrap
it in parentheses to make it a unary operation. This will be an error in future
versions of Sass.

More info and automated migrator: https://sass-lang.com/d/strict-unary
   ╷
31 │         margin: 0 -($spacer);
   │                 ^^^^^^^^^^^^
   ╵

Proposed resolution

Add rem unit to the value.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes and Update Helper on new feature change/addition
• ✅ Release Varbase 9.0.9, vartheme_bs4-9.0.18

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3305949 by RedwanJamous: Fixed invalid margin-bottom value for node__meta class