Remove the old timestamp selection

[ggevay]

The first commit removes the old timestamp selection, as discussed here: https://materializeinc.slack.com/archives/C08A62E0751/p1767794273757089?thread_ts=1764865885.225539&cid=C08A62E0751

The second commit updates a bunch of tests and fixes the error message when we don't find a suitable timestamp: Both the old and the new timestamp selection were relying on generate_timestamp_not_valid_error_msg, but that doesn't work well for the new timestamp selection for AS OF queries: the candidate comes out not as the AS OF time, so

SELECT * FROM t AS OF 1

would regress from

Timestamp (1) is not valid for all inputs: [u1]

to

Timestamp (1768481949666) is not valid for all inputs: []

which does not make sense: The [] is the list of inputs that this timestamp is not valid for, so we can see that actually this timestamp is valid for all inputs. But the problem here is not that this timestamp is not valid for an input, but the problem is the AS OF, which is not mentioned by the error msg.

So, the PR makes the error message print the constraints instead. It also tweaks the formatting of constraints a bit, to use more user-facing terminology, e.g., instead of ComputeInput we now say Indexed input. So, now we have stuff like:

select * from t AS OF 7;

ERROR:  could not find a valid timestamp for the query
DETAIL:  Constraints:
lower:
  (Storage inputs: [u1]): [1768492534743 (2026-01-15 15:55:34.743)]
upper:
  (Query's AS OF): [            7 (1970-01-01 00:00:00.007)]

Edit: I'm still fighting with a lot of annoying test output rewrites.

[def-]

I like removing code!

[def-]

We might want to keep this for older versions

[ggevay]

Yes, I've added it back.