Scope vaadin-grid header hiding to CronExpressionField only

[coderabbitai]

The CSS rule vaadin-grid::part(header) { display: none; } in cron-expression-field-styles.css is currently global and affects every vaadin-grid component in applications using CronExpressionField.

This selector should be scoped to only affect grids within CronExpressionField to prevent unintended side effects on other grids in the application.

Context:

• Issue identified during review of PR refactor: replace use of LumoUtility properties #16
• Comment: refactor: replace use of LumoUtility properties #16 (comment)
• Requested by: @javier-godoy

This is a pre-existing issue not introduced by PR #16.

[paodb]

There's only one grid in use in the component, and it's the "show next dates" grid which only shows one column that has no header, so there's no need to have any css rule to hide any grid header.