Update maven-javadoc-plugin to 3.1.1

[php-coder]

See

• 3.1.0: https://mail-archives.apache.org/mod_mbox/maven-announce/201903.mbox/%3C6064d830-474c-4b43-afef-99502c3a305a%40getmailbird.com%3E
• 3.1.1: https://mail-archives.apache.org/mod_mbox/maven-announce/201907.mbox/%3CCAPoyBqS_jmC_T%2Bg2jet6VoP-_CKFFtEZ-r_-EMvST%3D_RonWeCg%40mail.gmail.com%3E