Raise minimum Java version for jackson-annotations to Java 8 for Jackson 3.x

[cowtowncoder]

While all other Jackson 2.x components were changed to require Java 8 by Jackson 2.18, jackson-annotations has remained at Java/JDK 6 minimum.
While we need not change that for Jackson 2.x (no need or benefit), with Jackson 3.x baseline for other components will be raised to Java 17 or later (perhaps 21, see FasterXML/jackson-future-ideas#73). As a consequence, it may be impractical to retain Java 6 baseline.

So let's raise baseline to Java 8 for Jackson 3.0.