为什么项目没有部署到heroku？

我正在按照 heroku 上部署页面上的说明进行操作。但是在进入这一行（git push heroku master）之后，一切都崩溃了。错误本身：

   [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project myfoodserver: Fatal error compiling: invalid target release: 14 -> [Help 1]

我知道问题出在 maven 编译器上，但具体在哪里呢？我的 pom.xml：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.6.RELEASE</version>
    <relativePath/>
</parent>
<groupId>ru.denisov</groupId>
<artifactId>myfoodserver</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>myfoodserver</name>
<description>Server for MyFood app</description>

<properties>
    <java.version>14</java.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>

    <dependency>
        <groupId>org.postgresql</groupId>
        <artifactId>postgresql</artifactId>
        <version>42.2.12</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
        <exclusions>
            <exclusion>
                <groupId>org.junit.vintage</groupId>
                <artifactId>junit-vintage-engine</artifactId>
            </exclusion>
        </exclusions>
    </dependency>

    <dependency>
        <groupId>javax.xml.bind</groupId>
        <artifactId>jaxb-api</artifactId>
        <version>2.3.0</version>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>com.heroku.sdk</groupId>
            <artifactId>heroku-maven-plugin</artifactId>
            <version>3.0.2</version>
        </plugin>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>13</source>
                <target>13</target>
            </configuration>
        </plugin>
    </plugins>
</build>