databases/pg_partman: Update to 3.1.1

While here:

• Add LICENSE_FILE.
• Remove GH_TUPLE because it's been used for the default distribution file.
• Strip pg_partman_bgw.so (Q/A warning).
• Switch to options helper (DOCS).

Changes: https://github.com/keithf4/pg_partman/blob/v3.1.1/CHANGELOG.txt

PR: 225041
Submitted by: Lacey Powers <lacey.leanne@gmail.com> (maintainer)