go.mod: update logrus to v1.8.1

Newer versions contain fixes for recent Go versions, and this removes
the dependency on github.com/konsorten/go-windows-terminal-sequences

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/sirupsen/logrus/CHANGELOG.md

@@ -1,3 +1,39 @@
+# 1.8.1
+Code quality:
+  * move magefile in its own subdir/submodule to remove magefile dependency on logrus consumer
+  * improve timestamp format documentation
+
+Fixes:
+  * fix race condition on logger hooks
+
+
+# 1.8.0
+
+Correct versioning number replacing v1.7.1.
+
+# 1.7.1
+
+Beware this release has introduced a new public API and its semver is therefore incorrect.
+
+Code quality:
+  * use go 1.15 in travis
+  * use magefile as task runner
+
+Fixes:
+  * small fixes about new go 1.13 error formatting system
+  * Fix for long time race condiction with mutating data hooks
+
+Features:
+  * build support for zos
+
+# 1.7.0
+Fixes:
+  * the dependency toward a windows terminal library has been removed
+
+Features:
+  * a new buffer pool management API has been added
+  * a set of `<LogLevel>Fn()` functions have been added
+
 # 1.6.0
 Fixes:
   * end of line cleanup