fix(backdrop): remove blur effect for ionic theme (#30208)

Issue number internal

---------

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## What is the current behavior?
On the Ionic theme, ion-backdrop has a blur effect whenever it is
visible or hidden.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->
On the Ionic theme, ion-backdrop no longer has a blur effect regardless
of its state.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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  2. Update the BREAKING.md file with the breaking change.
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## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

core/src/components/backdrop/backdrop.ionic.scss

@@ -3,5 +3,4 @@
 
 :host {
   background-color: rgba(globals.$ion-primitives-base-black-rgb, 0.7);
-  backdrop-filter: blur(2px);
 }

core/src/components/modal/test/sheet/index.html

@@ -143,6 +143,12 @@
           >
             Backdrop is inactive
           </button>
+          <button
+            id="backdrop-hidden"
+            onclick="presentModal({ backdropBreakpoint: 0.5, initialBreakpoint: 0.3, breakpoints: [0.3, 0.5, 0.7, 1], showBackdrop: false })"
+          >
+            Backdrop is hidden
+          </button>
 
           <button
             id="half-sheet"

core/src/components/modal/test/sheet/modal.e2e.ts

@@ -47,6 +47,19 @@ configs({ modes: ['ionic-md'], directions: ['ltr'] }).forEach(({ title, screensh
       });
     });
   });
+
+  test.describe(title('sheet modal: backdrop hidden'), () => {
+    test('should not have visual regressions', async ({ page }) => {
+      await page.goto('/src/components/modal/test/sheet', config);
+      const ionModalDidPresent = await page.spyOnEvent('ionModalDidPresent');
+
+      await page.click('#backdrop-hidden');
+
+      await ionModalDidPresent.next();
+
+      await expect(page).toHaveScreenshot(screenshot(`modal-backdrop-hidden`), { animations: 'allow' });
+    });
+  });
 });
 
 configs({ modes: ['ios'], directions: ['ltr'] }).forEach(({ title, config }) => {