chore: publish 2.1.x through the Sonatype Central Portal

[ckawl]

main moved from Nexus 2 to the Central Portal in #1592. Nexus 2 is retired, and once a namespace has moved to the Central Portal it must not go back to OSSRH, so the 2.1.x release workflow cannot be used as it is.

This brings 2.1.x in line with main:

• release.yml is now identical to main's: Central Portal credentials and server id, inputs passed through env vars, and the POST that moves the staged deployment into the Central Portal for publishing
• pom.xml repositories and distributionManagement point at the OSSRH Staging API, as on main
• the archetype poms get a . Maven does not inherit name from the parent and the Central Portal rejects a deployment without one, which is what main hit in 730906a

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Description of changes:

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