Replace the jackson-base parent with a single self contained pom

[nrayburn-tech]

The jackson-base parent pom is only really intended for Jackson projects, so this puts the jackson-databind-nullable project in a better position to not inherit parent changes that may be undesirable.

This should be safe to do in 0.2.x, but it wouldn't hurt to wait until 0.3.x with some other breaking changes such as Jackson 3 support.
Switched this to just target 0.3.x and play it safe.

There are a few before/after changes, but I'm not sure there's anything that's relevant.

1. The MANIFEST.MF file uses "OpenAPI Tools" instead of "FasterXML" as the vendor.
2. The MANIFEST.MF file uses 8 instead of 1.8 for X-Compile-Source-JDK and X-Compile-Target-JDK.
3. Javadoc links point to https://openapitools.org/ instead of http://fasterxml.com/.
4. Some of the reporting plugins and configuration has been changed. I don't think this is relevant because this plugin doesn't use the site goal for anything that I'm aware of.
5. Maybe some others that I'm not seeing. I reviewed a diff of the effective pom before/after, a diff of the output of mvn verify before/after, and a diff of the output to the console before/after and the above is all I found that was relevant and changed.

Relevant parent pom file links.
https://repo1.maven.org/maven2/com/fasterxml/jackson/jackson-base/2.20.1/jackson-base-2.20.1.pom
https://repo1.maven.org/maven2/com/fasterxml/jackson/jackson-bom/2.20.1/jackson-bom-2.20.1.pom
https://repo1.maven.org/maven2/com/fasterxml/jackson/jackson-parent/2.20/jackson-parent-2.20.pom
https://repo1.maven.org/maven2/com/fasterxml/oss-parent/70/oss-parent-70.pom

[nrayburn-tech]

Currently the project just inherits the value from the parent pom, but this value should be updated when builds happen.

So what I have here matches existing behavior, but the existing behavior isn't really right.

[nrayburn-tech]

@wing328 I’m not sure if there’s any way to make this change easier to review. It is targeting the 0.3 branch, so anything that is broken will only be part of the new version with Jackson 3 support.