v0.12.6 : accurate timestamp formats in openapi, namespace exclusion list

Fixes the openapi conversion to reflect timestamp formats accurately
Adds an option (surfaced in CLI + sbt plugin) to exclude namespaces during code generation

What's Changed

• Update http4s-circe, http4s-client, ... to 0.22.12 by @scala-steward in #138
• Update jsoniter-scala-core to 2.13.7 by @scala-steward in #141
• openapi datetime mods by @yisraelU in #139
• Add a list of excluded namespaces by @daddykotex in #142

Full Changelog: v0.12.5...v0.12.6