Update release process in RELEASING.md (#6504)

Changes:
- Add a section on closing the milestone.
- Add a section on creating a `Version Release` issue.

Author: XSAM

.github/ISSUE_TEMPLATE/version_release.md

@@ -19,4 +19,4 @@ assignees: ''
 - [ ] [Sync with Contrib](https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/RELEASING.md#upgrade-goopentelemetryiootel-packages)
 - [ ] [Release contrib](https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/RELEASING.md#release-process)
 - [ ] [Sync website docs](https://github.com/open-telemetry/opentelemetry-go/blob/main/RELEASING.md#website-documentation)
-- [ ] Close the milestone
+- [ ] [Close the milestone](https://github.com/open-telemetry/opentelemetry-go/blob/main/RELEASING.md#close-the-milestone)

RELEASING.md

@@ -1,5 +1,9 @@
 # Release Process
 
+## Create a `Version Release` issue
+
+Create a `Version Release` issue to track the release process.
+
 ## Semantic Convention Generation
 
 New versions of the [OpenTelemetry Semantic Conventions] mean new versions of the `semconv` package need to be generated.
@@ -123,10 +127,24 @@ Importantly, bump any package versions referenced to be the latest one you just
 [Go instrumentation documentation]: https://opentelemetry.io/docs/languages/go/
 [content/en/docs/languages/go]: https://github.com/open-telemetry/opentelemetry.io/tree/main/content/en/docs/languages/go
 
+### Close the milestone
+
+Once a release is made, ensure all issues that were fixed and PRs that were merged as part of this release are added to the corresponding milestone.
+This helps track what changes were included in each release.
+
+- To find issues that haven't been included in a milestone, use this [GitHub search query](https://github.com/open-telemetry/opentelemetry-go/issues?q=is%3Aissue%20no%3Amilestone%20is%3Aclosed%20sort%3Aupdated-desc).
+- To find merged PRs that haven't been included in a milestone, use this [GitHub search query](https://github.com/open-telemetry/opentelemetry-go/pulls?q=is%3Apr+no%3Amilestone+is%3Amerged).
+
+Once all related issues and PRs have been added to the milestone, close the milestone.
+
 ### Demo Repository
 
 Bump the dependencies in the following Go services:
 
 - [`accounting`](https://github.com/open-telemetry/opentelemetry-demo/tree/main/src/accounting)
 - [`checkoutservice`](https://github.com/open-telemetry/opentelemetry-demo/tree/main/src/checkout)
 - [`productcatalogservice`](https://github.com/open-telemetry/opentelemetry-demo/tree/main/src/product-catalog)
+
+### Close the `Version Release` issue
+
+Once the todo list in the `Version Release` issue is complete, close the issue.