fix(modal): fix rounding issue in shouldShowScrollbar

[yinglejia]

shouldShowScrollbar uses scrollHeight to determine whether the modal content is scrollable or not. The problem is that scrollHeight returns a rounded value, and its implementation (round up or down) is browser-specific. For text elements, with the default Carbon styles, the modal content height is 28.02 px. Different browsers may return different values. e.g.:

• Chrome seems to always return 28.
• FireFox returns 28 for some languages (e.g. English), but returns 29 for some other languages (e.g. Chinese).

The previous implementation uses Math.round to round getBoundingClientRect().height before comparison. This results in false positive for some languages like Chinese. It seems we should use Math.ceil instead.

Before:

After:

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[carbon-bot]

:tada: This PR is included in version 4.59.11 :tada:

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[carbon-bot]

:tada: This PR is included in version 5.0.6 :tada:

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