Split EnvironmentsFinder into two

What does this MR do?

Before this MR EnvironmentsFinder has 2 different ways of use. There was a common execute method but there was also find method that worked completely differently (not shared code). Some classes used execute while others used find. This MR splits finder into two separate finders.

In details:

1. Move everything related to execute from app/finders/environments_finder.rb to app/finders/environments_by_deployments_finder.rb.
2. Move everything related to execute from spec/finders/environments_finder_spec.rb to spec/finders/environments_by_deployments_finder_spec.rb.
3. Classes that used EnvironmentsFinder#execute now use EnvironmentsByDeploymentsFinder#execute.
4. Classes that used EnvironmentsFinder#find are kept as is.

Step 1 from #326668 (closed)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is technical debt.
• 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