Require Go 1.15 or later

The library github.com/golang-jwt/jwt/v4 requires the FillByte() method
of *big.Int, so we're raising the minimum Go version to 1.15.

changelog/unreleased/pr-3680

@@ -0,0 +1,7 @@
+Change: Update dependencies and require Go 1.15 or newer
+
+We've updated most dependencies. Since some libraries require newer language
+features we're dropping support for Go 1.14 and restic now requires at least Go
+1.15 to build.
+
+https://github.com/restic/restic/issues/3680