databases/mongodb40-tools: Update to 4.0.21

• Pretty trivial version update.
• Point www to manual of 4.0.
• Update CONFLICTS_INSTALL. (I don't see how -tools conflicted with mongodb.)

Should have been part of r556769

PR: 556769
Submitted by: Ronald Klop
Approved by: dev.ashevchuk@gmail.com (maintainer timeout)