Add http semantic conventions notice to changelog (#6737)

Author: dmathieu

CHANGELOG.md

@@ -11,11 +11,23 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 This release is the last to support [Go 1.22].
 The next release will require at least [Go 1.23].
 
+> [!WARNING]
+> This is the last version to use Semantic Conventions v1.20.0 for HTTP libraries
+by default. The next version (0.61.0) will default to v1.26.0, and the
+following one (0.62.0) will drop support for Semantic Conventions v1.20.0
+>
+> You can switch to the new Semantic Conventions right now by setting the
+`OTEL_HTTP_CLIENT_COMPATIBILITY_MODE=http/dup` environment variable in your
+application.
+>
+> See also the [HTTP semantic conventions stability
+migration](https://github.com/open-telemetry/semantic-conventions/blob/main/docs/non-normative/http-migration.md)
+
 ### Added
 
 - Add support for configuring `ClientCertificate` and `ClientKey` field for OTLP exporters in `go.opentelemetry.io/contrib/config`. (#6378)
 - Add `WithAttributeBuilder`, `AttributeBuilder`, `DefaultAttributeBuilder`, `DynamoDBAttributeBuilder`, `SNSAttributeBuilder` to support adding attributes based on SDK input and output in `go.opentelemetry.io/contrib/instrumentation/github.com/aws/aws-sdk-go-v2/otelaws`. (#6543)
-- Support for the OTEL_HTTP_CLIENT_COMPATIBILITY_MODE=http/dup environment variable in `go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux` to emit attributes for both the v1.20.0 and v1.26.0 semantic conventions. (#6652)
+- Support for the `OTEL_HTTP_CLIENT_COMPATIBILITY_MODE=http/dup` environment variable in `go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux` to emit attributes for both the v1.20.0 and v1.26.0 semantic conventions. (#6652)
 - Added the `WithMeterProvider` option to allow passing a custom meter provider to `go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux`. (#6648)
 - Added the `WithMetricAttributesFn` option to allow setting dynamic, per-request metric attributes in `go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux`. (#6648)
 - Added metrics support, and emit all stable metrics from the [Semantic Conventions](https://github.com/open-telemetry/semantic-conventions/blob/main/docs/http/http-metrics.md) in `go.opentelemetry.io/contrib/instrumentation/github.com/gorilla/mux/otelmux`. (#6648)