switching from v1 to v2 of encoding/json in the root handler to not omit
false values in the spaceapi response.
this increases the min go version to 1.25.
Delete types/v14.go, replace with v15.go.
Rename internal types to not include message version.
I don't currently see the need to support multiple message
versions that have incompatible layouts.
v15 is backwards compatible with v14, so the config example now
specifies both.
Rename the "response" field in the config file to "static" as
dynamic/static makes more sense from the user's perspective.
(Internally it's still SpaceapidConfig.{Dynamic,Response}
to align with type names and usage.)
