Fixes bug, explained in issue#42627: when interval between requests smaller than rate interval, new tokens can not be added to bucket, because each request updates timer, and therefore rate interval never be reached.
I replace this with approach where timer updates to time, when should have been last updated, and only when rate interval reached.
I added test case for test adding tokens.
As far as I see, there is no BC and no deprecations.
There is method calculateNextTokenAvailability() on Symfony\Component\RateLimiter\Policy\Rate. It does not work correctly (does not match the name and the description), but as far as I can see, it is not used anywhere, so I left it unchanged. The Rate class simply cannot return this data, because it does not have a timer and does not know when the previous addition was made and, accordingly, what to calculate the next from.