go.mod: update github.com/cockroachdb/swiss for Go 1.25 support

[stefanb]

The Go 1.25 incompatibility found in

• go 1.25.0, go@1.24 1.24.6 (new formula) Homebrew/homebrew-core#226636

then:

• reported upstream in Enable Go 1.25 support swiss#48
• fixed upstream Enable support for Go 1.25 swiss#49

The changes in this PR are tested in my fork:

• This exact same PR: https://github.com/stefanb/pebble/pull/1
• With Go 1.25 in workflows: https://github.com/stefanb/pebble/pull/3

If desired I can bring some or all commits from https://github.com/stefanb/pebble/pull/3 into this PR to have this tested in CI with latest Go 1.25 (currently at rc1).

Once this is merged I suggest to create a release so that downstream users have plenty of time to include this before Go 1.25 is released (scheduled for August).

[cockroach-teamcity]

This change is 

[RaduBerinde]

Hi,
We use master for CRDB-specific development. We should do this on the pebble-release-2.0 branch. I will create a new release afterwards.

[stefanb]

Thanks, @RaduBerinde. As suggested, i have created a PR for the pebble-release-2.0 branch:

• go.mod: update github.com/cockroachdb/swiss to support Go 1.25 #4954

[RaduBerinde]

Thanks!