assorted (linting) fixes, dependency updates and test-fixes

[thaJeztah]

go.mod: remove toolchain directive

go.mod: update github.com/containerd/accelerated-container-image v1.2.x

Update the version of github.com/containerd/accelerated-container-image
to v1.2.0, which is the minimum version supporting containerd v2.

go.mod: update containerd dependency to stable v2.0.x

Updating to the v2.0.4 patch release as minimum, which includes
c8d@3d53430 to reduce some (indirect) dependencies, and c8d@f341477
to downgrade some unreleased versions of dependencies. The v2.0.4
release included a security fix for CVE-2024-40635, so updating to
that version to avoid potential security alerts. I did not update to
the latest version because this project is used as dependency for
other modules, and containerd v2.0.5 also updated the minimum Go version
to 1.23.

gofmt code

remove unused zdfsIsReady, pouchDirLock, blockEngine vars

These were added in the initial bc443fe
commit, but appear to be unused.

remove uses of deprecated io/ioutil

use filepath for joining file paths

On a Unix system, this doesn't make a big difference, but filepath.Join
is the preferred use for file paths (regardless if it's forward or backward
slashes). path.Join is still used for constructing URLs, which always use
forward slashes.

remove uses of github.com/pkg/errors in favor of stdlib

Use stdlib native error-wrapping so that we can remove the direct
dependency on github.com/pkg/errors.

update and fix tests

Update tests to use designated rfc6761 test domains; the tests didn't
require an actual domain to resolve; for such cases, it's generally
recommended to use rfc6761 designated domains.

Also:

• fix TestParse: the test was only printing the result, but didn't
  test it to be correct.
• use consts for fixed values
• remove uses of testify: the tests currently only used a very minimal
  subset of its functionality, so replacing it with stdlib to reduce
  the dependency tree.
• remove hard-coded temp-dir from test. Use t.TempDir() to provide
  the path and to clean up after the test if anything would fail.

replace direct uses of logrus for containerd/log

This allows consumers of this module to use the context-logger that's
defined through containerd's log package. If no logger is defined, it
uses logrus' standard logger.

[thaJeztah]

@BigVan @WaberZhuang PTAL

[thaJeztah]

@BigVan @WaberZhuang PTAL 🫶