Bump version to 1.27.1 (smithy-lang#1575)

Update 1.27.0 changelog

Bump version to 1.26.4 (smithy-lang#1510)

Amend changelog for 1.26.3

Bump version to 1.26.2

Bump version to 1.26.1 (smithy-lang#1476)

Revert "Allow transforms to remove enum members" (smithy-lang#1443)

This reverts commit b9d5a49.

Bump version to 1.25.2

Changelog and version bump for 1.25.1 release (smithy-lang#1422)

Update migration guide to reflect 1.25 changes