fix(docs): fix Python 3.8 SyntaxError in generate_msg_docs.py (#26788)

Line 319 used nested quotes inside f-strings, a feature only available
in Python 3.12+. The CI Docker image (px4-dev-base-focal:2021-08-18)
runs Python 3.8, causing the "msg file docs" Jenkins stage to fail on
every main build since 6bf73d9d89.

Extract the join expressions into local variables to restore
compatibility with Python 3.8+.

Tools/msg/generate_msg_docs.py

@@ -316,7 +316,9 @@ Param | Units | Range/Enum | Description
                 if val.minValue or val.maxValue:
                     rangeVal = f"[{val.minValue if val.minValue else '-'} : {val.maxValue if val.maxValue else '-' }]"
 
-                output+=f"{i} | {", ".join(val.units)}|{', '.join(f"[{e}](#{e})" for e in val.enums)}{rangeVal} | {val.description}\n"
+                units_str = ", ".join(val.units)
+                enums_str = ', '.join("[{}](#{})".format(e, e) for e in val.enums)
+                output+=f"{i} | {units_str}|{enums_str}{rangeVal} | {val.description}\n"
             else:
                 output+=f"{i} | | | ?\n"