Update substr method in text utilities

What does this MR do?

In the course of working on !18874 (merged), I noticed that the truncateSha function is using the legacy/non-recommended String.prototype.substr() method. This updates it to use String.prototype.substring()

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Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.