slog: rewrite level synchronization with Logrus

[thaJeztah]

slog: rewrite level synchronization with Logrus

Use the Logrus slog level adapter as a dynamic slog.Leveler instead of
maintaining a separate slog.LevelVar and duplicating the Logrus-to-slog
level mapping.

This keeps the Logrus logger as the single source of truth for the configured
level, so changes made through SetLevel or directly on L.Logger are reflected
by slog handlers automatically.

Also avoid formatting Logrus entries that are forwarded to slog and then
discarded.

slog: remove duplicate level mapping test

Remove TestLogrusToSlogLevel now that containerd uses the level adapter from
logrus/hooks/slog directly.

The mapping itself is covered by the Logrus package, while containerd's
integration tests verify that forwarded records use the expected slog levels
and that filtering follows the configured Logrus level.

slog: improve bridge test coverage

Verify that Logrus records are forwarded with the expected slog levels, and
that slog filtering follows changes to the Logrus logger level.

Cover both SetLevel and direct changes to L.Logger to ensure the dynamic level
adapter remains in sync without separate level state.

log: accept slog-compatible levels in SetLevel

Extend SetLevel to accept slog.Level and the level adapter types provided
by logrus/hooks/slog, in addition to strings and Level values.

Keep logrus.Level as the canonical representation. Map slog-backed levels
through logrus/hooks/slog, while accepting Logrus-backed levels directly.

Add coverage for all accepted level representations across the supported
log levels.

[thaJeztah]

Potentially this could also do the reverse, and use the Handler that logrus has if we want to keep logrus for the formatting;

slog.SetDefault(slog.New(
	logrusslog.NewHandler(L.Logger, nil),
))

[thaJeztah]

Noticed that we logrus was still configured to do the formatting, even it it wasn't used.

[thaJeztah]

Updated after #22 was merged; this now accepts both logrus and slog levels, as well as strings;

type levelValue interface {
    string | Level | slog.Level | lslog.Level | lslog.SlogLevel
}