Bug 1872954 - [Protections Panel] Ensure blocker tracker milestones subviewbutton is labeled. r=anti-tracking-reviewers,pbz

The Protections doorhanger UI includes [a subview button with id `protections-popup-milestones-content`](https://searchfox.org/mozilla-central/rev/762f24e00a9548d80ebba1b985c871ba6d9b829d/browser/components/controlcenter/content/protectionsPanel.inc.xhtml#160-167) that does not provide an accessible name,

It appears to happen because this `<toolbarbutton>` XUL control is marked up with [`<description>` as its child containing the on-screen text](https://searchfox.org/mozilla-central/rev/762f24e00a9548d80ebba1b985c871ba6d9b829d/browser/components/controlcenter/content/protectionsPanel.inc.xhtml#164-165), while `<label>` does provide an expected output for the Accessibility API preserving its visual appearance at the same time.

Since this control would become labeled and would start passing a11y_checks, we are also removing [the `fail-if` notation](https://searchfox.org/mozilla-central/rev/762f24e00a9548d80ebba1b985c871ba6d9b829d/browser/base/content/test/protectionsUI/browser.toml#57) from the `browser/base/content/test/protectionsUI/browser_protectionsUI_milestones.js` test manifest.

Note: the following prefs are needed:
```
      // Hide protections cards so as not to trigger more async messaging
      // when landing on the page.
      ["browser.contentblocking.report.monitor.enabled", false],
      ["browser.contentblocking.report.lockwise.enabled", false],
      ["browser.contentblocking.report.proxy.enabled", false],
      ["browser.contentblocking.cfr-milestone.update-interval", 0],
```

Differential Revision: https://phabricator.services.mozilla.com/D197634

Author: anna-yeddi

browser/base/content/test/protectionsUI/browser.toml

@@ -54,7 +54,6 @@ skip-if = ["a11y_checks"] # Bugs 1858041 and 1849179 for causing intermittent cr
 https_first_disabled = true
 
 ["browser_protectionsUI_milestones.js"]
-fail-if = ["a11y_checks"] # Bug 1854233 protections-popup-milestones-content may not be labeled
 
 ["browser_protectionsUI_open_preferences.js"]
 https_first_disabled = true

browser/components/controlcenter/content/protectionsPanel.inc.xhtml

@@ -161,8 +161,8 @@
                          class="subviewbutton"
                          flex="1"
                          oncommand="gProtectionsHandler.openProtections(true); gProtectionsHandler.recordClick('milestone_message');">
-            <description id="protections-popup-milestones-text" flex="1"
-                         role="heading" aria-level="2"/>
+            <label id="protections-popup-milestones-text" flex="1"
+                   role="heading" aria-level="2"/>
             <image id="protections-popup-milestones-illustration"/>
           </toolbarbutton>
         </hbox>