pgsql: Fix pg_event_trigger_ddl_commands for GRANT ON PROPERTY GRAPH

Fix pg_event_trigger_ddl_commands for GRANT ON PROPERTY GRAPH

stringify_grant_objtype() treated OBJECT_PROPGRAPH as unused, so
pg_event_trigger_ddl_commands() failed with "unsupported object type"
when a ddl_command_end trigger inspected GRANT/REVOKE on a property
graph. Return "PROPERTY GRAPH" like the GRANT command syntax.

Bug: #19637
Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Andrey Rachitskiy <pl0h0yp1(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/19637-4446f72945492ed8%40postgresql.org

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/63f8e9773aadba37661f70629a0a0e8f5933afc8

Modified Files
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src/backend/commands/event_trigger.c | 3 ++-
src/test/regress/expected/event_trigger.out | 21 ++++++++++++++++-----
src/test/regress/sql/event_trigger.sql | 11 ++++++++---
3 files changed, 26 insertions(+), 9 deletions(-)