Update data fetch strategy when variable updates

What does this MR do?

This is a cleanup MR 🧹

Currently, the VariablesSection component taps into Vuex actions twice to update variable values and then fetch data. It feels odd that the component has to do both of these. This MR updates it so that when VariablesSection updates variable values, the corresponding Vuex action responds and then triggers data fetch.

Concern

I have a concern that data fetch could become more magical and not very obvious. However, it feels odd that the component is responsible and also, I'm a fan of keeping things in one place.

Before

graph TB
  Component -- updateVariablesValues --> Actions
  Component -- fetchData --> Actions
  Actions --> Mutations
  Mutations --> Store
  Store --> Component

After

graph TB
  Component -- updateVariablesValues --> Actions
  Actions -- triggers fetchData --> Actions
  Actions --> Mutations
  Mutations --> Store
  Store --> Component

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team