Chase Bank is the only major institution with this problem. They need to update their website, not the other way around.
If you are so motivated, tell them to stop using the outdated and privacy-intrusive "user agent" query as the means to determine if a browser can handle their website, which was never a good idea to begin with.
Assuming you are not willing to take your business elsewhere, that leaves you out in the cold until they realize the error of their ways and correct it.
The workaround for end users who are unable to update their Macs to a newer macOS version is to use a different browser.
Consider Brave:
https://brave.com/
Firefox is a distant second choice:
https://www.mozilla.org/en-US/firefox/new/
The bottom line is that there is nothing wrong with your Mac's version of Safari, but that bank is using an outdated and ill-conceived method of determining browser requirements. I recommend that you direct their attention to the WWW publication User-Agent Client Hints (courtesy VikingOSX).
In-depth background discussion here: chase wants me to update safari to 16, should I do that? - Apple Community