Use `gl-dropdown` in Run Pipline form

What does this MR do and why?

Issue link: #363320 (closed)

This replaces the gl-form-select in the Run pipeline form with gl-dropdown. This only affects how the Variable Type dropdown looks like.

Functionality is still the same, so the original specs should still pass.

Screenshots or screen recordings

Behavior	Recording
Selecting variable type	form
Pre-filling with URL params	url_prefill

How to set up and validate locally

1. In your .gitlab-ci.yml, print the variable you want to use for the test. For example:

   build:
     stage: build
     script:
       - echo $FOO
   
   test:
     stage: test
     script:
       - echo $TEST

2. Go to CI/CD > Pipelines. Click the Run Pipeline button on the upper right corner of the page.
3. Observe that dropdown for the Variable Type field now uses the gl-dropdown component (instead of gl-form-select). It should have the Variable and File options.
4. Finish the form and run your pipeline. To validate that we ran the pipeline with the correct variable types for each variable, visit the job page for the job that uses those variables and find the line that prints your variable.

ENV variables will be printed as a string.

FILE variables will be printed as a file location.

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.