Changed atmosphere exposure constant (for non HDR) to make atmosphere brighter

[IanLilleyT]

Fixes #8149

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ 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.

[lilleyse]

@IanLilleyT can you add an entry to CHANGES.md?

Also just out of curiosity can you post the commit where this change was first introduced?

[IanLilleyT]

Added to CHANGES.md. The change was first introduced in 8cf28a6

[lilleyse]

👍