finagle-core: add writevAddresses to IOExceptionStrings

Problem:
finagle-thriftmux is failing in our github workflows
due to an unknown exception:

```Unknown exception closed channel.
io.netty.channel.unix.Errors$NativeIoException:
writevAddresses(..) failed: Broken pipe```

Solution:
Add `writevAddresses(..) failed: Broken pipe` to the
list of known exception strings

JIRA Issues: CSL-10921

Differential Revision: https://phabricator.twitter.biz/D661550

CHANGELOG.rst

@@ -13,6 +13,13 @@ Breaking API Changes
 * finagle-core: `c.t.f.param.Logger` has been removed. Use external configuration supported by
   your logging backend to alter settings of `com.twitter.finagle` logger.  ``PHAB_ID=D618667``
 
+Bug Fixes
+~~~~~~~~~~
+
+* finagle-core: Failed writes on Linux due to a remote peer disconnecting should now
+  be properly seen as a `c.t.f.ChannelClosedException` instead of a
+  `c.t.f.UnknownChannelException`. ``PHAB_ID=D661550``
+
 21.4.0
 ------
 

finagle-core/src/main/scala/com/twitter/finagle/IOExceptionStrings.scala

@@ -9,7 +9,8 @@ private[finagle] object IOExceptionStrings {
     "An existing connection was forcibly closed by the remote host", // Found on windows
     "syscall:read(..) failed: Connection reset by peer", // Found on linux w/ native epoll
     "readAddress(..) failed: Connection reset by peer", // Found on linux w/ native epoll
-    "writeAddress(..) failed: Connection reset by peer" // Found on linux w/ native epoll
+    "writeAddress(..) failed: Connection reset by peer", // Found on linux w/ native epoll
+    "writevAddresses(..) failed: Broken pipe" // Found on linux w/ native epoll
   )
 
   /** Strings that commonly signal failure to establish a socket connection */