[envvars-api] update oapi-codegen
- add installation of oapi-codegen to makefile
- remove github.com/deepmap/oapi-codegen from go.mod file
- add github.com/oapi-codegen/echo-middleware to go.mod file
- update neccessary files
Bug: T362290 Signed-off-by: Ndibe Raymond Olisaemeka rolisaemeka-ctr@wikimedia.org