Add doc on how to upgrade to new semconv (open-telemetry#7807)

Adds a new "Semantic Conventions Upgrade" section to CONTRIBUTING.md
that documents the complete process for upgrading to newer semantic
conventions releases as requested in issue open-telemetry#7697.

It includes the details regarding:

- Generating new semconv packages
- Updating imports across the codebase
- Updating the opentelemetry-go-contrib linter configuration

---------

Co-authored-by: Damien Mathieu <42@dmathieu.com>
Co-authored-by: Robert Pająk <pellared@hotmail.com>

Author: 3 people

RELEASING.md

@@ -4,7 +4,9 @@
 
 Create a `Version Release` issue to track the release process.
 
-## Semantic Convention Generation
+## Semantic Convention Upgrade
+
+### Semantic Convention Generation
 
 New versions of the [OpenTelemetry Semantic Conventions] mean new versions of the `semconv` package need to be generated.
 The `semconv-generate` make target is used for this.
@@ -22,6 +24,43 @@ make semconv-generate # Uses the exported TAG.
 This should create a new sub-package of [`semconv`](./semconv).
 Ensure things look correct before submitting a pull request to include the addition.
 
+The `CHANGELOG.md` should also be updated to reflect the new changes:
+
+```md
+- The `go.opentelemetry.io/otel/semconv/<NEW VERSION>` package. The package contains semantic conventions from the `<NEW VERSION>` version of the OpenTelemetry Semantic Conventions. See the [migration documentation](./semconv/<NEW VERSION>/MIGRATION.md) for information on how to upgrade from `go.opentelemetry.io/otel/semconv/<PREVIOUS VERSION>`. (#PR_NUMBER)
+```
+
+> **Tip:** Change to the release and prior version to match the changes
+
+### Update semconv imports
+
+Once the new semconv module has been generated, update all semconv imports throughout the codebase to reference the new version:
+
+```go
+// Before
+semconv "go.opentelemetry.io/otel/semconv/v1.37.0"
+"go.opentelemetry.io/otel/semconv/v1.37.0/otelconv"
+
+
+// After
+semconv "go.opentelemetry.io/otel/semconv/v1.39.0"
+"go.opentelemetry.io/otel/semconv/v1.39.0/otelconv"
+```
+
+Once complete, run `make` to check for any compilation or test failures.
+
+#### Handling attribute changes
+
+Some semconv releases might add new attributes or impact attributes that are currently being used. Changes could stem from a simple renaming, to more complex changes like merging attributes and property values being changed.
+
+One should update the code to the new attributes that supersede the impacted ones, hence sticking to the semantic conventions. However, legacy attributes might still be emitted in accordance to the `OTEL_SEMCONV_STABILITY_OPT_IN` environment variable.
+
+For an example on how such migration might have to be tracked and performed, see issue [#7806](https://github.com/open-telemetry/opentelemetry-go/issues/7806).
+
+### Go contrib linter update
+
+Update [.golangci.yml](https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/.golangci.yml) in [opentelemetry-go-contrib](https://github.com/open-telemetry/opentelemetry-go-contrib/) to mandate the new semconv version.
+
 ## Breaking changes validation
 
 You can run `make gorelease` which runs [gorelease](https://pkg.go.dev/golang.org/x/exp/cmd/gorelease) to ensure that there are no unwanted changes made in the public API.