Update sbt-coveralls to 1.3.15

Author: scala-steward

project/plugins.sbt

@@ -1,6 +1,6 @@
 addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "1.1.4")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.3.0")
-addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.3.14")
+addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.3.15")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.7")
 addSbtPlugin("com.github.sbt" % "sbt-github-actions" % "0.24.0")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.4")