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