fix(modal): set correct backdrop opacity for ionic theme (#29960)

Issue number: resolves internal

---------

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## What is the current behavior?
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The backdrop opacity is incorrectly set to 0.7

## What is the new behavior?
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this PR. -->

Sets backdrop opacity to 1

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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[Preview](https://ionic-framework-pkqwthhvj-ionic1.vercel.app/src/components/modal/test/sheet?ionic:theme=ionic)

Author: tanner-reits

core/src/components/modal/modal.ionic.scss

@@ -7,7 +7,8 @@
 :host {
   --background: #{globals.$ionic-color-base-white};
   --box-shadow: #{globals.$ionic-elevation-300};
-  --backdrop-opacity: 0.7;
+  // Backdrop opacity is 1 because the backdrop's background color has an alpha value
+  --backdrop-opacity: 1;
 
   color: globals.$ionic-color-neutral-1200;
 }