Maven统一管理依赖的版本号之统一配置spring的版本

使用<properties>标签，如下：

    <properties>
        <spring.version>5.2.6.RELEASE</spring.version>
    </properties>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring.version}</version>
        </dependency>

    </dependencies>