The dependencyLicenses check has the ability to map multiple jar files
to the same license file. However, netty was not taking advantage of
this, and had duplicate copies of its license/notice files for each jar.
This commit reduces the copies to one and uses the mapping feature.
We kept `netty_3` as a fallback in the 5.x series but now that master
is 6.0 we don't need this or in other words all issues coming up with
netty 4 will be blockers for 6.0.
This commit upgrades the transport-netty4 module dependency from Netty
version 4.1.5 to version 4.1.6. This is a bug fix release of Netty.
Relates #21051
This commit upgrades the Netty dependencies from version 4.1.4 to
version 4.1.5. This upgrade brings several bug fixes including the
removal of a obnoxious and scary-looking log message when unsafe is
explicitly disabled.
Relates #20222
