startsWith is not supported on IE11

[OmarShehata]

The PR #8758 added a startsWith call, which is not supported on IE11.

This PR replaces it with an indexOf() check. Tested with a built Cesium Viewer.

@mramato

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[mramato]

Confirmed everything now works in IE11 as expected.