chore: remove 2.12.8 and 2.12.9 publishing (#508)

If  someone is updating to the newest scoverage and are still on
2.12.8 or 2.12.9 there is no reason for them to not be able to bump
their Scala version. There is no need to keep publishing all of these.
I hit on this in publishing since scalameta wasn't published for 2.12.8
and the release failed because of it.

Author: ckipp01

Diff for: bin/test-release.sh

@@ -4,8 +4,6 @@ set -eux
 version=$1
 
 coursier fetch \
-  org.scoverage:scalac-scoverage-plugin_2.12.8:$version \
-  org.scoverage:scalac-scoverage-plugin_2.12.9:$version \
   org.scoverage:scalac-scoverage-plugin_2.12.10:$version \
   org.scoverage:scalac-scoverage-plugin_2.12.11:$version \
   org.scoverage:scalac-scoverage-plugin_2.12.12:$version \

Diff for: build.sbt

@@ -15,9 +15,7 @@ lazy val bin212 =
     "2.12.13",
     "2.12.12",
     "2.12.11",
-    "2.12.10",
-    "2.12.9",
-    "2.12.8"
+    "2.12.10"
   )
 lazy val bin213 =
   Seq(