Update parent pom and core requirement

[alecharp]

Kind of a follow up of jenkinsci/jenkins#5320.

ping @car-roll

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[car-roll]

2.303 does seem like a big bump, but it does seem like the majority of users are on that.

[jglick]

FTR: jenkinsci/jenkins#5320 is first in 2.303.x as an LTS line, so this is not arbitrary.

[bitwiseman]

The 2.303.x line is the current LTS line, so I'd generally rather not force people to it, but there aren't other changes to be released. This seems fine to me.

[car-roll]

Yikes, I accidentally bumped jenkins to 2.304 in #120, fixing here and reflecting change in logs

[jglick]

Kind of a follow up of jenkinsci/jenkins#5320.

@alecharp @car-roll can you explain in what sense this is a follow-up to that core change? This baseline is causing me some problems in handling #172 so I would like to know if it was truly necessary or if I can in fact revert the baseline to, say, 2.289.x at least for one release.