Update golang version to 1.11.5

**What this PR does / why we need it**:

> We have just released Go 1.11.5 and Go 1.10.8 to address a recently reported security issue. We recommend that all users update to one of these releases (if you’re not sure which, choose Go 1.11.5).
>
> This DoS vulnerability in the crypto/elliptic implementations of the P-521 and P-384 elliptic curves may let an attacker craft inputs that consume excessive amounts of CPU.
>
> These inputs might be delivered via TLS handshakes, X.509 certificates, JWT tokens, ECDH shares or ECDSA signatures. In some cases, if an ECDH private key is reused more than once, the attack can also lead to key recovery.
>
> The issue is CVE-2019-6486 and Go issue golang/go#29903. See the Go issue for more details.

K8S issue kubernetes/kubernetes#73238

**Which issue(s) this PR fixes**:
n/a

**Special notes for your reviewer**:

**Release note**:
<!--  Write your release note:
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2. If no release note is required, just write "NONE" within the block.

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```improvement operator
NONE
```

/cc @ThormaehlenFred

Dockerfile

@@ -1,5 +1,5 @@
 #############      builder       #############
-FROM golang:1.11.4 AS builder
+FROM golang:1.11.5 AS builder
 
 WORKDIR /go/src/github.com/gardener/gardener-extensions
 COPY . .