"a software update is required to connect to your iPhone." is a potential hack?

That prompt for the "a software update is required to connect to your iPhone." is quite normal and is unrelated to the problems you later encountered. Just coincidence.

And by the way, third party anti-virus software is a waste of money and slows down the performance of Macs. macOS is quite capable of taking care of itself with its built-in protections, malware scanning, gatekeeper protection, and several more built-in security blocks. Consider installing Bit Defender, it’s really quite useless on a modern Mac.

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

So uninstall BitDefender AntiVirus according to the developer's instructions.

If you're sure the message was from a text message and not generated by your Mac just ignore it.

Predicated upon what the User Wrote " On a Macbook Air M1, I received a unsolicited text message with an image. Soon after, I was prompted with the "a software update is required to connect to your iPhone." - Both my iPhone 12 and my mac have the latest updates, Firewall is on and I have Bitdefender Antivirus. "

An " unsolicited text message " implies a message in the Messages Application from an unknown cellphone number. I receive those all the time and more often that not they are s pure Scams especially when links of Images are included.

Then there is the BitDefender aspect.

There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

Additionally, and specific to Big Sur macOS 11 and Monterey macOS 12, the Operating System resides in a Sealed and Read Only Volume that can not be opened by the User and protects against from “ Bad Actor “ Software.

The Only thing this Antivirus software is protecting is the Bank Account of the Developers and for zero return to the User aside from the problems this software creates.

AntiVirus Developers purposefully Market their product to create a “ Fear Factor “. In so doing, creating a False Need for their Product.

The The Built in Security  is all that is required to protect the computer.

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster and comprising the Operating System

Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a portion on each sale. What the Application may do to the computer is up to the User to check this out before purchase

Any of the below should be removed as per Developers Instructions

This will include BitDefender and strongly suggest reading some of the posting specific to this software and arrive at your own conclusions

Scam and praying upon the innocence of the user

refer to below from Apple and the Official Way this should work

https://support.apple.com/en-ca/HT208831

FWIW, there is a known bug connecting an iPhone running iOS 16 to a Mac running Monterey 12.6 that brings up that message. Don't know when it will be fixed.