go.mod: update github.com/cockroachdb/swiss to support Go 1.25
The Go 1.25 incompatibility found in
* Homebrew/homebrew-core#226636
then:
* reported upstream in cockroachdb/swiss#48
* fixed upstream cockroachdb/swiss#49

db: fix obsolete file metric underflows
Previously a race existed in the calculation of obsolete file metrics resulting
in underflow. The values within versionSet.metrics were reset and recalculated
to reflect the set of files in versionSet.obsoleteTables whenever new files
were added to obsoleteFiles. Additionally, the cleanup manager invoked a
callback to decrease versionSet.metrics whenever a table was deleted.
The recalculation of versionSet.metrics could reset the metrics to less than
the sum of outstanding pending deletes. When the cleanup manager eventually
deleted the pending tables, these metrics would underflow.
This commit fixes the bug by maintaining separate stats for all files that have
been enqueued for the cleanup manager and all files that have been successfully
deleted. The volume of outstanding, pending deletions is the difference between
the two.
For now, there's an additional wart that the set of files that are sitting in
versionSet.obsoleteTables are still separately tracked.
Informs #4811.

go.mod: update deps to support go1.24

go.mod: update github.com/prometheus/client_golang to v1.15.0

sstable: update testutil readall to return an error
ReadAll is importing testutils which is pulling testify and other
test dependencies in the final binary
update function to return an error without calling testutils

pebble-release-2.0: change path to v2
A go module with version `v2.0` must have a path that ends in `/v2`;
otherwise `go get` gives you a "module path must match major version"
error.
This commit enacts this change for the `pebble-release-2.0` branch. In
the future we will consider making this change on master as well so we
don't have inconsistency between the branches.
Fixes #4271

pebble-release-1.1: db: fix Open issue with single-item relative paths
Previously Open with a relative path containing a single element would fail on
real filesystems, because Open would attempt to open the empty path.
Fix #3842.

metamorphic: skip non-suffixed keys on time-bound iterators
The metamorphic tests sometimes generates iterators with block-property filters
constraining the set of MVCC timestamps that should be observed. Previously
keys that were not suffixed were guaranteed to always be surfaced. The testkeys
block-property collector would map an unsuffixed key to the maximal interval,
ensuring any block containing an unsuffixed key would be visited by all
time-bound iterators.
The cockroachdb block-property collector does not give non-MVCC keys the same
treatment. Instead, it simply ignores non-timestamped keys for the purpose of
calculating block-property collectors. This results in non-deterministic
presence of non-MVCC keys in a time-bound iterators iteration. With the
introduction of cockroach kvs in the metamorphic test, we'll need consistent,
deterministic behavior. Skipping non-MVCC keys on time-bound iterators ensures
the iterator observes the same keys regardless of the behavior of the
underlying block property collector.

README: add information about v2 and format upgrade

tool: add db upgrade command
Add a `db upgrade` command that ratches up the DB version to the
newest one.