chore(readme): remove experimental wording (#27398)

Issue number: N/A

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## What is the current behavior?
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We've been using the custom elements bundle in Ionic for a few years
now. The underlying behavior is stable in Stencil, so we should remove
the "experimental" verbiage from the README.


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

- Removed experimental wording 
## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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