Update logback-classic, logback-core to 1.5.15 (#706)

## About this PR
📦 Updates 
* [ch.qos.logback:logback-classic](https://github.com/qos-ch/logback)
* [ch.qos.logback:logback-core](https://github.com/qos-ch/logback)

 from `1.5.13` to `1.5.15`

## Usage
✅ **Please merge!**

I'll automatically update this PR to resolve conflicts as long as you
don't change it yourself.

If you'd like to skip this version, you can just close this PR. If you
have any feedback, just mention me in the comments below.

Configure Scala Steward for your repository with a
[`.scala-steward.conf`](https://github.com/scala-steward-org/scala-steward/blob/fa7b6604b226097c532b4655814fa62a46ce546d/docs/repo-specific-configuration.md)
file.

_Have a fantastic day writing Scala!_

<details>
<summary>⚙ Adjust future updates</summary>

Add this to your `.scala-steward.conf` file to ignore future updates of
this dependency:
```
updates.ignore = [ { groupId = "ch.qos.logback" } ]
```
Or, add this to slow down future updates of this dependency:
```
dependencyOverrides = [{
  pullRequests = { frequency = "30 days" },
  dependency = { groupId = "ch.qos.logback" }
}]
```
</details>

<sup>
labels: test-library-update, early-semver-patch, semver-spec-patch,
commit-count:1
</sup>

<!-- scala-steward = {
  "Update" : {
    "ForGroupId" : {
      "forArtifactIds" : [
        {
          "ForArtifactId" : {
            "crossDependency" : [
              {
                "groupId" : "ch.qos.logback",
                "artifactId" : {
                  "name" : "logback-classic",
                  "maybeCrossName" : null
                },
                "version" : "1.5.13",
                "sbtVersion" : null,
                "scalaVersion" : null,
                "configurations" : "test"
              }
            ],
            "newerVersions" : [
              "1.5.15"
            ],
            "newerGroupId" : null,
            "newerArtifactId" : null
          }
        },
        {
          "ForArtifactId" : {
            "crossDependency" : [
              {
                "groupId" : "ch.qos.logback",
                "artifactId" : {
                  "name" : "logback-core",
                  "maybeCrossName" : null
                },
                "version" : "1.5.13",
                "sbtVersion" : null,
                "scalaVersion" : null,
                "configurations" : "test"
              }
            ],
            "newerVersions" : [
              "1.5.15"
            ],
            "newerGroupId" : null,
            "newerArtifactId" : null
          }
        }
      ]
    }
  },
  "Labels" : [
    "test-library-update",
    "early-semver-patch",
    "semver-spec-patch",
    "commit-count:1"
  ]
} -->

project/Dependencies.scala

@@ -30,7 +30,7 @@ object Dependencies {
   }
 
   object Logback {
-    private val version = "1.5.13"
+    private val version = "1.5.15"
     val core            = "ch.qos.logback" % "logback-core"    % version
     val classic         = "ch.qos.logback" % "logback-classic" % version
   }