I am encountering an issue regarding the PHP update.
On the control panel, I am indeed on version 8.3, but on the dashboard of the WordPress site there is a message telling me the site is on version 7.3…?!
How should I proceed to update to the latest version?
The most important thing is that you check which is the latest and compatible PHP version for your theme. If you haven't added any additional plugins, it shouldn't cause any issues when you spin up your environment. If you encounter any problem, do what @Gaston suggested: deactivate all plugins and then reactivate them one by one to pinpoint the possible issue.
Leave a SINGLE .ovhconfig in your hosting. This one could be generated when you go to your client area, hosting section and choose the execution environment.
Alternatively, if you have multiple sites in your multisite, and they require different PHP versions from each other, you can place a .ovhconfig in the main folder of each sub‑site, and in that case you must not place a .ovhconfig in the root of your hosting.
The error message you have: “GET not implemented” occurs when the wrong value “stable” is used instead of “stable64” or vice‑versa in the .ovhconfig file. You must use “stable” up to php7.3 and “stable64” from php7.4.