chore(deps): update dependency apache-beam to v2.20.0 (#3411)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [apache-beam](https://beam.apache.org) | minor | `==2.19.0` -> `==2.20.0` |

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dataflow/encryption-keys/requirements.txt

@@ -1 +1 @@
-apache-beam[gcp]==2.19.0
+apache-beam[gcp]==2.20.0

dataflow/flex-templates/streaming_beam/requirements.txt

@@ -1 +1 @@
-apache-beam[gcp]==2.19.0
+apache-beam[gcp]==2.20.0

pubsub/streaming-analytics/requirements.txt

@@ -1 +1 @@
-apache-beam[gcp,test]==2.19.0
+apache-beam[gcp,test]==2.20.0