[postgresqlreceiver] bump supported PG version to 12 (open-telemetry#…

…35898)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Update the prerequisites requirements for using this receiver (+
integration tests) to the latest officially supported PostgreSQL version

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes open-telemetry#30923

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Unit & integration tests

<!--Describe the documentation added.-->
#### Documentation
Updated README

<!--Please delete paragraphs that you did not use before submitting.-->

.chloggen/bump_minimal_pg_version.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: deprecation
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: postgresqlreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Minimal supported PostgreSQL version updated from 9.6 to 13.0
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [30923]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: Aligning on the supported versions as can be seen [in the PostgreSQL releases section](https://www.postgresql.org/support/versioning)
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]