### Context
While doing the balancing of the Balance Sheets, I found that:
1. some OSS accounts were being created with non-sensical account codes (e.g. in Lithuania, the OSS account is created with code 4402 whereas the basic VAT Payable account has code 4492), 
2. some CoAs already have a dedicated account for OSS (e.g. Germany should use account 1767 for OSS), but we create a new one.

### Problem 1: non-sensical account codes
At heart, this is due to the implementation of `_search_new_account_code`. What it should do, is start with the given `prefix`, expand it to `digits`, and increment it until an available code is found. 

However, this is not what `_search_new_account_code` does at the moment: if `len(prefix) == digits`, it truncates the last digit of `prefix`, then appends 1, 2, 3, ... until an available code is found.
This behaviour is unwanted for two reasons:
- Firstly, if `prefix` ends with a different digit than `0` or `1`, we want to only try codes higher than `prefix`.
- Secondly, the resulting code may exceed the expected number of digits passed to it. This was ruled by TSB as something that should be avoided, due to constraints on the length of codes in some countries (e.g. limited to 8 in Germany).

We therefore change the behaviour of `_search_new_account_code` to the following:
1. _search_new_account_code now takes an argument `start_code`, which is a code to start incrementing from, instead of `prefix` and `digits`.
2. We check whether `start_code` is available; if not, we increment it until an available code is found. The incrementation works by regexing an integer at the end of the `start_code` string, and incrementing it without increasing the length of the code.
3. If, due to non-numeric characters in the code, the incrementation fails, or if no codes are available, we fallback to `'{start_code}.copy'`, `'{start_code}.copy2'`, etc.


Because `_search_new_account_code` is used to create new account codes when loading CoA templates, this change in behaviour (in particular, ensuring that the generated code has the specified number of digits) requires us as well to fix too-low values of the `code_digits` property on CoA templates. 

I also note that while the implementation of `AcccountAccount.copy()` does not currently use `_search_new_account_code`, we could consider using it at some point. 

### Problem 2: CoAs that already provide an OSS account
We need a way to specify an existing account from the CoA if there is one dedicated for OSS. For this, I've just added the CoA accounts directly in the `l10n_eu_oss` module, as I didn't think providing a prefix in each localization's template was needed. However, if that's considered better, I'm open to the change.

Enterprise PR: https://github.com/odoo/enterprise/pull/48066

taskids: 3521121, 3523224