[8.14](backport #3834) Add link to workaround about using custom CAs in private locations

[mergify]

Closes #3833.

Someone who is an SME should make sure this note is accurate and in the right place. Synthetics is not my area of expertise; I'm just trying to help out.

IMO pointing to that GitHub issue may not be the best approach because the details in the issue seem speculative. It would be much better if we could be proscriptive and tell users exactly how to work around the problem. That is beyond of the scope of what was requested in #3833 and would need some effort from folks outside of the docs team.

TODO (after merging):

• Port this change to serverless.

  ---

  This is an automatic backport of pull request Add link to workaround about using custom CAs in private locations #3834 done by Mergify.

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