build(deps): bump golang (#4174)

Bumps golang from 1.20-alpine to 1.21-alpine.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

instrumentation/github.com/bradfitz/gomemcache/memcache/otelmemcache/example/Dockerfile

@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-FROM golang:1.20-alpine AS base
+FROM golang:1.21-alpine AS base
 COPY . /src/
 WORKDIR /src/instrumentation/github.com/bradfitz/gomemcache/memcache/otelmemcache/example