Changes needed before gokit/log to slog transition. (#39527)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** Resolves #38889

PLEASE READ BELOW before looking at file changes

Before converting individual files/packages to slog, we generally need
to make these 2 changes to make the conversion easier:
- Replace uses of `kitlog.With` since they are not fully compatible with
our kitlog adapter
- Directly use the kitlog adapter logger type instead of the kitlog
interface, which will let us have direct access to the underlying slog
logger: `*logging.Logger`

Note: that I did not replace absolutely all uses of `kitlog.Logger`, but
I did remove all uses of `kitlog.With` except for these due to
complexity:
- server/logging/filesystem.go and the other log writers (webhook,
firehose, kinesis, lambda, pubsub, nats)
- server/datastore/mysql/nanomdm_storage.go (adapter pattern)
- server/vulnerabilities/nvd/* (cascades to CLI tools)
- server/service/osquery_utils/queries.go (callback type signatures
cascade broadly)
- cmd/maintained-apps/ (standalone, so can be transitioned later all at
once)

Most of the changes in this PR follow these patterns:
- `kitlog.Logger` type → `*logging.Logger`
- `kitlog.With(logger, ...)` → `logger.With(...)`
- `kitlog.NewNopLogger() → logging.NewNopLogger()`, including similar
variations such as `logging.NewLogfmtLogger(w)` and
`logging.NewJSONLogger(w)`
- removed many now-unused kitlog imports

Unique changes that the PR review should focus on:
- server/platform/logging/kitlog_adapter.go: Core adapter changes
- server/platform/logging/logging.go: New convenience functions
- server/service/integration_logger_test.go: Test changes for slog

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
  - Was added in previous PR

## Testing

- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Refactor**
* Migrated the codebase to a unified internal structured logging system
for more consistent, reliable logs and observability.
* No user-facing functionality changed; runtime behavior and APIs remain
compatible.
* **Tests**
* Updated tests to use the new logging helpers to ensure consistent test
logging and validation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

ee/server/integrationtest/scim/suite.go

@@ -5,9 +5,9 @@ import (
 	"testing"
 
 	"github.com/fleetdm/fleet/v4/server/fleet"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/fleetdm/fleet/v4/server/service"
 	"github.com/fleetdm/fleet/v4/server/service/integrationtest"
-	"github.com/go-kit/log"
 )
 
 type Suite struct {
@@ -22,7 +22,7 @@ func SetUpSuite(t *testing.T, uniqueTestName string) *Suite {
 	ds, fleetCfg, fleetSvc, ctx := integrationtest.SetUpMySQLAndService(t, uniqueTestName, &service.TestServerOpts{
 		License: license,
 	})
-	logger := log.NewLogfmtLogger(os.Stdout)
+	logger := logging.NewLogfmtLogger(os.Stdout)
 	users, server := service.RunServerForTestsWithServiceWithDS(t, ctx, ds, fleetSvc, &service.TestServerOpts{
 		License:     license,
 		FleetConfig: &fleetCfg,