Fix reading agent version

[SylvainJuge]

What does this PR do?

• For agent version 1.27.0, the agent is unable to read its own version
• This is a small regression introduced with Isolated agent classloader #2109
• (minor) when service_version is not set, the agent will produce a (null) in the logs, which could be confusing.

Has been reported in the forum here.

For a not-yet-known reason, reading maven descriptor as resources does not seem to work as expected
with the new agent architecture where classes and resources are moved to a separate /agent folder to exclude them from classpath.

However, reading the META-INF/MANIFEST.MF works as expected as this file remains in the same location in the agent jar itself, so this PR just implements this minor change.

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

[apmmachine]

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• Start Time: 2021-11-25T15:03:29.562+0000

• Duration: 72 min 17 sec

• Commit: 76c44e5

Test stats 🧪

Test	Results
Failed	0
Passed	2548
Skipped	21
Total	2569

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