Fix pom.xml files in the examples so that they can run with a more re…

[jovo3]

…cent JDK

Error seen when running the examples:
Source option 5 is no longer supported. Use 6 or later.

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Checklist

Testing

(Remove this checklist and replace it with "N/A - no code changes" if this PR does not modify source code)

• I have manually verified that my code changes do the right thing.
• I have run the tests and verified that my changes do not introduce any regressions.
• I have written unit tests to verify that my code changes do the right thing and to protect my code against regressions

Documentation

(Remove this checklist and replace it with "N/A - no code changes" if this PR does not modify source code)

• I have updated the "Unreleased" section of CHANGELOG.md with a brief description of my changes.
• I have updated code comments - both GroovyDoc/JavaDoc-style comments and inline comments - where appropriate.
• I have read CONTRIBUTING.md and have followed its guidance.

[awittha]

Please parameterize the plugin version into <properties>.

For an example, see how the gmavenplus plugin's version is parameterized here:

jenkins-spock/examples/helper-script/pom.xml

Line 31 in dc26b35

<groovy.gmaven.pluginVersion>1.6.1</groovy.gmaven.pluginVersion>