Rename SCSS calc height variables to meaningful names

What does this MR do and why?

In order to simplify the addition of a new logged out header in !125252 (closed) update SCSS variables to clarify actual usage, and reduce number of files changed in logged out header MR !125252 (closed)

Rename SCSS calc height variables to meaningful names

• Rename existing $calc-application-bars-height to $calc-system-bars-height (system header and performance bar)
• Update calc(#{$header-height} + #{$calc-system-bars-height}) usage to new $calc-application-bars-height variable

Screenshots or screen recordings

How to set up and validate locally

1. Enable system header in admin settings
2. Toggle the performance bar with p + b
3. Toggle new navigation
   1. Sidebar should be after application bars
   2. Issue sidebar should be after application bars
4. Toggle old navigation
   1. Issue sidebar should be after application bars

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #412613 (closed)