fix: resolve remaining staticcheck linting issues

- Remove newline characters from error messages in step.go
- Change capitalized error messages to lowercase in multienv_step_runner.go and github_client.go

server/core/config/raw/step.go

@@ -211,12 +211,12 @@ func (s Step) Validate() error {
 		case []interface{}:
 			for _, e := range t {
 				if _, ok := e.(string); !ok {
-					return fmt.Errorf("%q step %q option must contain only strings, found %v\n",
+					return fmt.Errorf("%q step %q option must contain only strings, found %v",
 						stepName, ShellArgsArgKey, e)
 				}
 			}
 		default:
-			return fmt.Errorf("%q step %q option must be a string or a list of strings, found %v\n",
+			return fmt.Errorf("%q step %q option must be a string or a list of strings, found %v",
 				stepName, ShellArgsArgKey, t)
 		}
 		delete(argMap, ShellArgsArgKey)

server/core/runtime/multienv_step_runner.go

@@ -37,7 +37,7 @@ func (r *MultiEnvStepRunner) Run(
 
 		vars, err := parseMultienvLine(res)
 		if err != nil {
-			return "", fmt.Errorf("Invalid environment variable definition: %s (%w)", res, err)
+			return "", fmt.Errorf("invalid environment variable definition: %s (%w)", res, err)
 		}
 
 		for i := 0; i < len(vars); i += 2 {

server/events/vcs/github_client.go

@@ -966,11 +966,11 @@ func (g *GithubClient) MergePull(logger logging.SimpleLogging, pull models.PullR
 
 		isMethodAllowed, isMethodExist := mergeMethodsAllow[method]
 		if !isMethodExist {
-			return fmt.Errorf("Merge method '%s' is unknown. Specify one of the valid values: '%s'", method, strings.Join(mergeMethodsName, ", "))
+			return fmt.Errorf("merge method '%s' is unknown. Specify one of the valid values: '%s'", method, strings.Join(mergeMethodsName, ", "))
 		}
 
 		if !isMethodAllowed() {
-			return fmt.Errorf("Merge method '%s' is not allowed by the repository Pull Request settings", method)
+			return fmt.Errorf("merge method '%s' is not allowed by the repository Pull Request settings", method)
 		}
 	} else {
 		method = defaultMergeMethod