BUG #16645: pg_upgrade does not mark template1 as a system database

The following bug has been logged on the website:

Bug reference:      16645
Logged by:          Y. NAKAGAWA
Email address:      nakagawa@goodf.co.jp
PostgreSQL version: 13.0
Operating system:   Windows 10 64bit
Description:

Repro steps:
1. Install postgresql-12.4-1-windows-x64.exe and
postgresql-13.0-1-windows-x64.exe.

2. Edit pg_hba.conf's on both servers to be able to connect to the servers
without password.

3. Open command prompt with administrator privilege and type:
cd %HOMEDRIVE%%HOMEPATH%
"C:\Program Files\PostgreSQL\13\bin\pg_upgrade.exe" -d "C:\Program
Files\PostgreSQL\12\data" -D "C:\Program Files\PostgreSQL\13\data" -b
"C:\Program Files\PostgreSQL\12\bin" -B "C:\Program Files\PostgreSQL\13\bin"
-U postgres

4. Open pgAdmin 4, connect to the PostgreSQL 13.0 server and expand the
Databases node.

Expected result:
The node has [postgres].

Actual result:
The node has [postgres] and [template1].