An excess contribution that is present in your HSAs can be applied as part or all of your contribution for any future year provided that you are eligible to make an HSA contribution for that year. So yes, as long as you are eligible to contribute at least $700 to an HSA for 2023, you can apply the $700 excess from 2021 as part of your 2023 contribution. The $700 will appear on line 43 of your 2023 Form 5329. You can also contribute additional amounts bringing your total contributions (including the applied excess) up to your contribution limit for 2023.
If you are an eligible individual on December 31, 2023 you can make a total contribution up to the full-year limit under the last-month rule. However, if you don't plan to remain an eligible individual throughout 2024, you might want to limit your HSA contribution for 2023 to only the annual limit prorated for the number of moths that you are an eligible individual. Otherwise, I'm not sure why you would not want to contribute the maximum permissible unless you lack the funds to do so.
