refactor: input value getters and setters (CP: 14)

[mukherjeesudebi]

Description

Backports the following PRs:

• refactor: move input value getters and setters to InputMixin web-components#5586

Part of vaadin/web-components#5763

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✅ vursen
❌ mukherjeesudebi
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[vursen]

The unit test failures on SauceLabs don't seem to be connected to this PR, so I think it's safe to merge.