Drop support for Go 1.20 (#4967)

* Update README.md

* Remove 1.20 support from CI workflows

* Update all go mod

* Add changelog entry

* Update go mod tidy target

* Run go mod tidy

* Replace sliceEqualFunc with slices.EqualFunc

* Replace grow with slices.Grow

* Replace ensureAttributesCapacity with slices.Grow

* Replace conditional with min

* Use slices module for slice comparison in metricdatatest

metric/go.mod

@@ -1,6 +1,6 @@
 module go.opentelemetry.io/otel/metric
 
-go 1.20
+go 1.21
 
 require (
 	github.com/stretchr/testify v1.8.4

metric/go.sum

@@ -6,6 +6,7 @@ github.com/go-logr/logr v1.4.1/go.mod h1:9T104GzyrTigFIr8wt5mBrctHMim0Nb2HLGrmQ4
 github.com/go-logr/stdr v1.2.2 h1:hSWxHoqTgW2S2qGc0LTAI563KZ5YKYRhT3MFKZMbjag=
 github.com/go-logr/stdr v1.2.2/go.mod h1:mMo/vtBO5dYbehREoey6XUKy/eSumjCCveDpRre4VKE=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=