Fix version selector for parity with previous version

[BSFishy]

Description

The current version selector navigates to <url>/<version> instead of <url>/<version>/. The trailing / is necessary to navigate to the correct version. Currently, if you try navigating to a different version on https://oui.opensearch.org/1.4/, it won't change the version.

Additionally, I made a change to the way the response promise is handled to properly handle case where there is an error with the fetch.

I think a change was made to the CDK to fix this as well, but I wanted to keep parity with the previous version of the version selector

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [x] All tests pass
  • [x] yarn lint
  • [x] yarn test-unit
• [ ] Update CHANGELOG.md
• [x] Commits are signed per the DCO using --signoff

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