I found the name a bit vague in trying to understand what this code was
doing. This updates it to lowest_ts_to_retain(), and updates the
disk_space_manager to use it rather than duplicating the logic.

target_excess -> adjusted_target_excess: I find the name more intuitive,
despite being more verbose at call sites, as a reader I prefer the
reminder that the value has been tweaked

I found the existing logging a bit confusing because "tiered storage
retention" isn't widely used verbiage. This tweaks it to refer to this
as "space management of tiered storage topics".

Also s/recover/remove: it feels a bit clearer, segment removal isn't
referred to as "recovering" in other contexts.

The previous name is pretty vague: as a reader it's unclear what it's
actually doing other than modifying the config in some way.

Updates it to explicitly mention that it is handling local storage
overrides. IMO this is easier to reason about when reading the method
and its call sites.

Similar to maybe_override_retention_config(), updates
override_retention_config() to be called
apply_local_storage_overrides().

Renames apply_base_overrides() to apply_kafka_retention_overrides().
I think it's a bit easier to infer what the Kafka retention overrides
are (i.e. the ones that are Kafka compatible: retention.ms,
retention.bytes).

Others considered but not taken:
- total_retention
- full_retention
- consumable_retention
- topic_retention

Pulls out a couple instances of duplicate code, in an effort to clarify
naming of various overrides and adjustments done around retention.

With the original code, it was easy to get tripped up by the nots and
ors. Tweaks the implementation and adds a comment to make this easier to
read.

No functional changes.

We have seen issues where the lack of tiered storage resulted in disk
filling up. The info logs didn't make it clear what was happening and
it was assumed to be a bug in code, rather than a misconfiguration
issue. This commit bumps a log line to info level so that it would have
been clear that a specific error path wasn't being taken.

Also tweaks wording to makes a stronger connection to tiered storage.