Vendor missing dependencies with govendor add +external

[pdecat]

This PR adds the mongodb dependencies introduced by #53 that are not vendored:

pagerduty/cache.go:13:2: cannot find package "go.mongodb.org/mongo-driver/bson" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/bson (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/bson (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/bson (from $GOPATH)
pagerduty/cache.go:14:2: cannot find package "go.mongodb.org/mongo-driver/bson/primitive" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/bson/primitive (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/bson/primitive (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/bson/primitive (from $GOPATH)
pagerduty/cache.go:15:2: cannot find package "go.mongodb.org/mongo-driver/mongo" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/mongo (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/mongo (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/mongo (from $GOPATH)
pagerduty/cache.go:16:2: cannot find package "go.mongodb.org/mongo-driver/mongo/options" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/mongo/options (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/mongo/options (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/mongo/options (from $GOPATH)
pagerduty/cache.go:17:2: cannot find package "go.mongodb.org/mongo-driver/mongo/readpref" in any of:
	/home/runner/work/go-pagerduty/go-pagerduty/src/github.com/heimweh/go-pagerduty/vendor/go.mongodb.org/mongo-driver/mongo/readpref (vendor tree)
	/opt/hostedtoolcache/go/1.16.7/x64/src/go.mongodb.org/mongo-driver/mongo/readpref (from $GOROOT)
	/home/runner/work/go-pagerduty/go-pagerduty/src/go.mongodb.org/mongo-driver/mongo/readpref (from $GOPATH)

Also, what about switching from govendor to go modules to manage dependencies?

[pdecat]

No longer needed since switch to go modules.