ICU-22561 Added maven-gpg-plugin in pom.xml to sign artifacts for mav…

[yumaoka]

…en central release.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22561
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[yumaoka]

This updated pom.xml was actually used for 74.1 release. Because repo was already tagged at that time, I merged the change to main branch at that time. For 74.2 maintenance update, we need this one merged to maint-74 branch as discussed in 2023-11-30 ICU-TC meeting.

[markusicu]

Please retarget this cherry-pick PR to maint/maint-74.

Given that it currently targets main, how is it possible that it shows the same change as in PR #2693 which you merged into main??

Did you follow the instructions or do something equivalent?

[mihnita]

Yes, main contains this already: https://github.com/unicode-org/icu/blob/main/icu4j/pom.xml#L484

[yumaoka]

@markusicu @mihnita My mistake. Retargetted to maint-74.

[markusicu]

rslgtm tnx

[mihnita]

Thanks!
LGTM (but for some reason)

For some reason I don't get the buttons to approve.
So consider this an approval.