Update release notes and version number to 1.8.1 (#4370)

This PR updates release notes and version number to 1.8.1
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

coremain/version.go

@@ -2,7 +2,7 @@ package coremain
 
 // Various CoreDNS constants.
 const (
-	CoreVersion = "1.8.0"
+	CoreVersion = "1.8.1"
 	coreName    = "CoreDNS"
 	serverType  = "dns"
 )

notes/coredns-1.8.1.md

@@ -15,9 +15,25 @@ The CoreDNS team has released
 
 ## Brought to You By
 
+Blake Ryan, Bob, Chotiwat Chawannakul, Chris O'Haver, Guangwen Feng, Jiang Biao, Johnny Bergström, Matt Kulka, Miek Gieben, Serge, Yong Tang, ZouYu, mgugger
+
 ## Noteworthy Changes
 
 * plugin/{clouddns,azure,route53}: Use cancelable contexts for cloud provider plugin refreshes (https://github.com/coredns/coredns/pull/4226)
 * plugin/health: Fix health check endpoint (https://github.com/coredns/coredns/pull/4231)
 * plugin/local: add local plugin (https://github.com/coredns/coredns/pull/4262)
-* plugin/kubernetes: plugin/kubernetes: Watch EndpointSlices (https://github.com/coredns/coredns/pull/4209)
+* plugin/kubernetes: Watch EndpointSlices (https://github.com/coredns/coredns/pull/4209)
+* plugin/dnstap: remove custom encoder (https://github.com/coredns/coredns/pull/4242)
+* plugin/dnstap: remove config struct (https://github.com/coredns/coredns/pull/4258)
+* plugin/local: add local plugin (https://github.com/coredns/coredns/pull/4262)
+* pkg/tls: remove InsecureSkipVerify=true flag (https://github.com/coredns/coredns/pull/4265)
+* plugin/trace: Fix zipkin json_v2 (https://github.com/coredns/coredns/pull/4180)
+* plugin/azure: return FQDN as MNAME in SOA record (https://github.com/coredns/coredns/pull/4286)
+* plugin/kubernetes: Fix dns programming duration metric (https://github.com/coredns/coredns/pull/4255)
+* plugin/file: Use NXDOMAIN response if CNAME target is NXDOMAIN (https://github.com/coredns/coredns/pull/4303)
+* plugin/cache: Move .LocalAddr() out of goroutine (https://github.com/coredns/coredns/pull/4281)
+* plugin/forward: respond with REFUSED when max_concurrent is exceeded (https://github.com/coredns/coredns/pull/4326)
+* core: custom DoH request validation (https://github.com/coredns/coredns/pull/4329)
+* plugin/kubernetes: Fix NPE issue (https://github.com/coredns/coredns/pull/4338)
+* plugin/kubernetes: Add support for dual stack ClusterIP Services (https://github.com/coredns/coredns/pull/4339)
+* plugin/azure: Iterate over all RecordSetListResultPage Pages (https://github.com/coredns/coredns/pull/4351)