fix(GlDropdown): Dropdown icon only fix with textSrOnly

Problem

Dropdowns that contain icon, text, text-sr-only, and no-caret don't receive the dropdown-icon-only class, which results in extra right padding. There are only a few edge cases where this exists, but it could more in the future, since most of these can be defined in the component properties themselves, resulting in cleaner code. This makes it possible to not have to use a template #button-content:

No template:

<gl-dropdown
      icon="ellipsis_v"
      :text="dropdownText"
      :text-sr-only="true"
      no-caret
      right
    >

Same output, but using a template and needing to import GlIcon:

<gl-dropdown
      no-caret
      right
    >
      <template #button-content>
        <gl-icon name="ellipsis_v" />
        <span class="gl-sr-only">{{ dropdownText }}</span>
      </template>

Before	After

Where in the product do these scenarios exist?

WIP: Issue actions dropdown	Diff settings dropdown (you can see the extra padding between the icons)

Does this MR meet the acceptance criteria?

Conformity

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Security

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
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