[receiver/googlecloudpubsub] add deprecation for build-in encoders (open-telemetry#39842)

#### Description
Add deprecation warning for the build-in encoders. The build-in encoders
`cloud_logging` and `raw_text` both have encoding extension alternatives
and will be removed in version v0.132.0 of the collector

#### Link to tracking issue
Fixes open-telemetry#39371

#### Testing
Started the collector with the encoders selected and see the warning

#### Documentation
Added deprecation to the README

Author: alexvanboxel

.chloggen/googlecloudpubsubreceiver-deprecate-buildin.yaml

@@ -0,0 +1,13 @@
+change_type: deprecation
+
+component: googlecloudpubsubreceiver
+
+note: Add deprecation warning for the build-in encoders
+
+issues: [39371]
+
+subtext: |
+  The build-in encoders `cloud_logging` and `raw_text` both have encoding extension alternatives and will be removed
+  in version v0.132.0 of the collector.
+
+change_logs: [user]