chore(deps): update module mvdan.cc/gofumpt to v0.9.0 (#7292)

This PR contains the following updates:

| Package | Change | Age | Confidence |
|---|---|---|---|
| [mvdan.cc/gofumpt](https://redirect.github.com/mvdan/gofumpt) |
`v0.8.0` -> `v0.9.0` |
[![age](https://developer.mend.io/api/mc/badges/age/go/mvdan.cc%2fgofumpt/v0.9.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/go/mvdan.cc%2fgofumpt/v0.8.0/v0.9.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

> [!WARNING]
> Some dependencies could not be looked up. Check the Dependency
Dashboard for more information.

---

### Release Notes

<details>
<summary>mvdan/gofumpt (mvdan.cc/gofumpt)</summary>

###
[`v0.9.0`](https://redirect.github.com/mvdan/gofumpt/blob/HEAD/CHANGELOG.md#v090---2025-09-02)

[Compare
Source](https://redirect.github.com/mvdan/gofumpt/compare/v0.8.0...v0.9.0)

This release is based on Go 1.25's gofmt, and requires Go 1.24 or later.

A new rule is introduced to "clothe" naked returns for the sake of
clarity.
While there is nothing wrong with naming results in function signatures,
using lone `return` statements can be confusing to the reader.

Go 1.25's `ignore` directives in `go.mod` files are now obeyed;
any directories within the module matching any of the patterns
are now omitted when walking directories, such as with `gofumpt -w .`.

Module information is now loaded via Go's [`x/mod/modfile`
package](https://pkg.go.dev/golang.org/x/mod/modfile)
rather than executing `go mod edit -json`, which is way faster.
This should result in moderate speed-ups when formatting many
directories.

</details>

---

### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined),
Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you
are satisfied.

♻ **Rebasing**: Whenever PR is behind base branch, or you tick the
rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update
again.

---

- [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check
this box

---

This PR was generated by [Mend Renovate](https://mend.io/renovate/).
View the [repository job
log](https://developer.mend.io/github/open-telemetry/opentelemetry-go).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0MS45MS4xIiwidXBkYXRlZEluVmVyIjoiNDEuOTEuMSIsInRhcmdldEJyYW5jaCI6Im1haW4iLCJsYWJlbHMiOlsiU2tpcCBDaGFuZ2Vsb2ciLCJkZXBlbmRlbmNpZXMiXX0=-->

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: dmathieu <damien.mathieu@elastic.co>

exporters/otlp/otlptrace/otlptracehttp/mock_collector_test.go

@@ -145,15 +145,16 @@ func (c *mockCollector) getInjectHTTPStatus() int {
 	return status
 }
 
-func (c *mockCollector) getInjectResponseHeader() (h map[string]string) {
+func (c *mockCollector) getInjectResponseHeader() map[string]string {
+	var h map[string]string
 	if len(c.injectResponseHeader) == 0 {
-		return
+		return h
 	}
 	h, c.injectResponseHeader = c.injectResponseHeader[0], c.injectResponseHeader[1:]
 	if len(c.injectResponseHeader) == 0 {
 		c.injectResponseHeader = nil
 	}
-	return
+	return h
 }
 
 func readRequest(r *http.Request) ([]byte, error) {