Bump version to 1.9.1, and add release notes (#5247)

This PR bumps coredns to 1.9.1 and add release notes.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

coremain/version.go

@@ -2,7 +2,7 @@ package coremain
 
 // Various CoreDNS constants.
 const (
-	CoreVersion = "1.9.0"
+	CoreVersion = "1.9.1"
 	coreName    = "CoreDNS"
 	serverType  = "dns"
 )

notes/coredns-1.9.1.md

++++
+title = "CoreDNS-1.9.1 Release"
+description = "CoreDNS-1.9.1 Release Notes."
+tags = ["Release", "1.9.1", "Notes"]
+release = "1.9.1"
+date = "2022-03-09T00:00:00+00:00"
+author = "coredns"
++++
+
+This is a release with security and bug fixes and some new features added. 1.9.1 is also built
+with golang 1.17.8 that addressed several golang 1.17.6 vulnerabilities (CVE-2022-23772,
+CVE-2022-23773, CVE-2022-23806).
+Note golang 1.17.6 was used to built coredns 1.9.0.
+
+## Brought to You By
+
+Chris O'Haver,
+Elijah Andrews,
+Rudolf Schönecker,
+Yong Tang,
+nathannaveen,
+xuweiwei
+
+## Noteworthy Changes
+
+* plugin/autopath: Don't panic on empty token (https://github.com/coredns/coredns/pull/5169)
+* plugin/cache: Add zones label to cache metrics (https://github.com/coredns/coredns/pull/5124)
+* plugin/file: Add TXT test case (https://github.com/coredns/coredns/pull/5079)
+* plugin/forward: Don't panic when from-zone cannot be normalized (https://github.com/coredns/coredns/pull/5170)
+* plugin/grpc: Fix healthy proxy error case (https://github.com/coredns/coredns/pull/5168)
+* plugin/grpc: Don't panic when from-zone cannot be normalized (https://github.com/coredns/coredns/pull/5171)
+* plugin/k8s_external: Implement zone transfers (https://github.com/coredns/coredns/pull/4977)
+* plugin/k8s_external: Fix external nsAddrs when CoreDNS Service has no External IPs (https://github.com/coredns/coredns/pull/4891)
+* plugin/kubernetes: Log api connection failures and server start delay (https://github.com/coredns/coredns/pull/5044)
+* plugin/log: Expand `{combined}` and `{common}` in log format (https://github.com/coredns/coredns/pull/5230)
+* plugin/metrics: Add metric counting DNS-over-HTTPS responses (https://github.com/coredns/coredns/pull/5130)
+* plugin/reload: Change hash from md5 to sha512 (https://github.com/coredns/coredns/pull/5226)
+* plugin/secondary: Fix startup transfer failure wrong zone logged (https://github.com/coredns/coredns/pull/5085)