Backend: Use CI/CD UI variables to define parallelism
### Update
See https://gitlab.com/gitlab-org/gitlab/-/issues/11549#note_2692859329
### Problem
The number of desired parallel jobs is oftentimes calculated using a script, As a user, I would like to automatically define that number as the number of parallel jobs instead of configuring it manually each time
### Proposal
To reduce the manual intervention can support variable expansion in [parallel](https://docs.gitlab.com/ee/ci/yaml/#parallel) so users could automatically calculate and update the number of parallel jobs
```yaml
Drupal 8 Audits:
stage: Run Audits
parallel: ${D8_PARALLEL_JOBS}
...
```
<details><summary> Original ask</summary>
We are trying to define variables using the UI to control the number of parallel streams for our CI runs.
CI Lint can't seem to parse the variables correctly as integer type and consequently fails at the first hurdle.
```
Drupal 8 Audits:
stage: Run Audits
parallel: ${D8_PARALLEL_JOBS}
...
Status: syntax is incorrect
Error: jobs:drupal 8 audits parallel is not a number
```
Is there a way to be able to pass the parallelism in as a variable?
</details>
issue