Improve modal with tertiary icon button for close action

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### Summary

<!-- Summarize the bug encountered concisely. -->

The modal component could be improved with a tertiary icon button for the close action. This will yield the result that the close action has a dedicated background upon interaction. This improves the intractability now that the tertiary button component is ready for use.

### Steps to reproduce

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1. Open a modal at for example [/feature_flags](https://gitlab.com/gitlab-org/gitlab/-/feature_flags) by pressing the configure feature flags button
1. Hover the close `X` button in the top right corner
1. See almost no difference in visual interaction

### Example Project

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behavior, and link to it here in the bug report. If you are using an older version of GitLab, this 
will also determine whether the bug is fixed in a more recent version. -->

Open the configure feature flags modal at [/feature_flags](https://gitlab.com/gitlab-org/gitlab/-/feature_flags) by pressing the configure feature flags button.

### What is the current *bug* behavior?

<!-- Describe what actually happens. -->

See almost no difference in visual interaction

### What is the expected *correct* behavior?

<!-- Describe what you should see instead. -->

See a clear visual change upon interaction

### Relevant logs and/or screenshots

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 as it's tough to read otherwise. -->

| gif of interaction with current close button of a modal  |
| --- |
| ![Screenshot_2020-11-05_at_12.58.34](/uploads/db26b9847238f7d3ff74ac530936a9c8/Screenshot_2020-11-05_at_12.58.34.gif) |

| Gif of visual change on interaction with tertiary buttons  |
| --- |
| ![Screenshot_2020-11-05_at_12.59.24](/uploads/66099f4218ed4a0e23bd79b4e0c9df2d/Screenshot_2020-11-05_at_12.59.24.gif) |

### Output of checks

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This bug happens on GitLab.com

#### Results of GitLab environment info

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<details>
<summary>Expand for output related to GitLab environment info</summary>

<pre>

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

</pre>
</details>

#### Results of GitLab application Check

<!--  Input any relevant GitLab application check information if needed. -->

<details>
<summary>Expand for output related to the GitLab application check</summary>
<pre>

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

</pre>
</details>

### Possible fixes

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Story points: 1.0