feat(card): convert card-content to shadow DOM

[brandyscarney]

Issue number: internal

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What is the current behavior?

Card content has no encapsulation.

What is the new behavior?

Convert ion-card-content to Shadow DOM which improves consistency among components & CSP compatibility.

Does this introduce a breaking change?

• [x] Yes
• [ ] No

There should not be any breaking changes related to targeting inner elements since ion-card-content does not have any internal elements of its own. However, some user styles may break due to the removal of the card-content-{mode} class or changes in selector specificity.