New Syrian flag and United Arab Republic

[lmoureaux]

Describe the bug
Our flag of Syria is the one used by the United Arab Republic from 1958 to 1961 and Syria from 1980 to 2024. The de-facto new flag is the so-called "Independence Flag".

To Reproduce
Steps to reproduce the behavior:

1. Check our Syrian flag

Expected behavior
The Syrian flag reflects the current situation.

The current flag can be reused for a new nation, the United Arab Republic.

While at it, we may also consider adding the Federation of Arab Republics.

Platform and version (please complete the following information):

• OS: Linux
• Freeciv21 version: 3.1-rc.1
• Ruleset/Longturn game (if applicable): all

Additional context
See https://en.wikipedia.org/wiki/Flag_of_Syria