Migrate publishing logic from OSSRH to Central Portal

[GuntherRademacher]

These changes replace the outdated OSSRH publishing configuration in pom.xml by the new settings for the Central Portal. The profile was consequently renamed from ossrh to central.

As prerequisites,

• the private key cg@basex.org must be present in the gpg keyring (use gpg --list-secret-keys to verify)

• the basex-publish user token for Central Portal must be present in ~/.m2/settings.xml, e.g.

  <?xml version="1.0" encoding="UTF-8"?>
  <settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0
            http://maven.apache.org/xsd/settings-1.0.0.xsd">
    <servers>
      <server>
        <id>basex-publish</id>
        <username>actual username</username>
        <password>actual password</password>
      </server>
    </servers>
  </settings>

Publishing can then be done fully automated, with MAVEN_GPG_PASSPHRASE set to the actual passphrase for the cg@basex.org key, by running

mvn clean deploy -Pcentral

Versions 12.1 through 12.3 have now been published using this approach (note that this publishing was done using org.sonatype.central:central-publishing-maven-plugin:0.7.0, while this PR moves forward to ...:0.10.0).

In addition to the pom.xml changes, two Javadoc warnings have been fixed, which were reported in the build log.

Fixes #2649