[SUREFIRE-1955] Switch project to Java 8

[slawekjaranowski]

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[Tibor17]

Usually, we split this activity to multiple activities.

1. changing the Maven parent POM and Java Version 1.8 in POM.
2. Changing the Maven API version
3. changing the code towards Java 8 (there is one plugin which converts the code to java 8 features)

[slawekjaranowski]

This PR only change Java version to 1.8 with only necessary changes to make project build.
So it is for 1 only from your list.

Maven Api was changed in #412

[olamy]

Good job

[Tibor17]

@slawekjaranowski
The build process is currently using two JDKs. One is 1.8 and the other is in the range [1.7, ) for tests.
This movement towards 1.8 would simplify and finally only one JDK will be used.
So please go to the Jenkinsfile and remove jdkTestName and rename it with jdkName, and final String jdkName = jenkinsEnv.jdkFromVersion(os, '8')
and substitute
final String jdkTestName = jenkinsEnv.jdkFromVersion(os, jdk.toString())
with
final String jdkName = jenkinsEnv.jdkFromVersion(os, jdk.toString()).
Then remove def script = cmd + ['\"-DjdkHome=$JAVA_HOME_IT\"'] and def script = cmd + ['\"-DjdkHome=%JAVA_HOME_IT%\"'] and rename script.join(' ') with cmd.join(' ').

Then remove the following in maven-failsafe-plugin/pom.xml:

                        <configuration>
                            <javaHome>${jdkHome}</javaHome>
                        </configuration>

Pls remove -DjdkHome=... from README.md.
Remove <jvm>${jdkHome}/bin/java</jvm> and <property>jdkHome</property> from the main POM.
Remove <jvm>${jdkHome}/bin/java</jvm> from the surefire-its/pom.xml

[Tibor17]

@slawekjaranowski
The plugin animal-sniffer-maven-plugin has a newer version 1.20. It is used to check the JDK API signatures. We can update the version here or in another PR. It's up to you.

[slawekjaranowski]

animal-sniffer-maven-plugin in next PR, there is one #411 - can be reopened after it