[Spring] config 'templateDirectory' for custom mustache files is not working for language Spring for file api.mustache

Description

with codegen maven plugin version 3.0.29 the configuration parameter 'templateDirectory' for custom mustache files is not working for language 'Spring' for file api.mustache.
However the same works with pojo.mustache in the specified templateDirectory.

OS: windows 10

It throws a FileNotFoundException in URLTemplateLoader of handlebars-4.1.2.jar. Cause: it prefixes the correct resolved absolute path with "/handlebars/JavaSpring/

e.g.
Caused by: java.io.FileNotFoundException: /handlebars/JavaSpring/C:/Users/.../templates/api.mustache

Swagger-codegen version

3.0.29

Swagger declaration file content or url

plugin configuration

<plugin>
    <groupId>io.swagger.codegen.v3</groupId>
        <artifactId>swagger-codegen-maven-plugin</artifactId>
        <version>3.0.29</version>
        <executions>
            <execution>
                <goals>
                    <goal>generate</goal>
                </goals>
                <configuration>
                    <inputSpec>
                        /src/main/resources/specification.yaml
                    </inputSpec>
                    <language>spring</language>
                    <apiPackage>org.myproject.gen.api</apiPackage>
                    <modelPackage>org.myproject.gen.model</modelPackage>
                    <generateModels>false</generateModels>
                    <configOptions>
                        <dateLibrary>java8</dateLibrary>
                        <interfaceOnly>true</interfaceOnly>
                    </configOptions>
                    <typeMappings>
                        <typeMapping>Double=java.math.BigDecimal</typeMapping>
                    </typeMappings>
                    <supportingFilesToGenerate>
                        HomeController.java,SwaggerUiConfiguration.java
                    </supportingFilesToGenerate>
                    <templateDirectory>src/main/resources/templates
                    </templateDirectory>
                </configuration>
            </execution>
        </executions>
    </plugin>

Related issues/PRs

similar issues:
https://github.com/swagger-api/swagger-codegen/issues/8132
https://github.com/swagger-api/swagger-codegen/issues/8132

Start with the Spring generator's handling of api.mustache and the handlebars 4.1.2 URLTemplateLoader path shown in the report; compare it with the working pojo.mustache lookup. Reproduce using the Maven plugin configuration and templateDirectory on Windows, then verify that api.mustache loads from the configured directory without the malformed /handlebars/JavaSpring/ prefix.