[encoding/googlecloudlogentryencoding] Update stability to alpha. (open-telemetry#39840)

#### Description
Last PR of a new component: Bring the `googlecloudlogentryencoding`
extension to Alpha

#### Link to tracking issue
open-telemetry#37531

Author: alexvanboxel

.chloggen/googlecloudlogentryencodingextension_alpha.yaml

@@ -0,0 +1,10 @@
+change_type: new_component
+
+component: googlecloudlogentryencodingextension
+
+note: Update stability to alpha.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37531]
+
+change_logs: [user]