Add Let's Encrypt errors to 12.5.5 changelog

Why is this change being made?

Make it more visible to users that the Let's Encrypt challenge error is now fixed. This hit many on the community channels, e.g. https://forum.gitlab.com/t/lets-encrypt-error/32357/8

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what