Bump haproxy version to 2.8.11

[CFN-CI]

Automatic bump from version 2.8.10 to version 2.8.11, downloaded from https://www.haproxy.org/download/2.8/src/haproxy-2.8.11.tar.gz.

After merge, consider releasing a new version of haproxy-boshrelease.

[maxmoehl]

There is one set of changes in the CHANGELOG that could cause issues:

    - BUG/MINOR: h1: Fail to parse empty transfer coding names
    - BUG/MINOR: h1: Reject empty coding name as last transfer-encoding value
    - BUG/MEDIUM: h1: Reject empty Transfer-encoding header

We need to check the changes more closely.

[peanball]

Verified that any behavioural change in HAProxy does not impact apps, as Gorouter that sits behind HAProxy was already checking the Transfer-Encoding header with strict enough rules that this issue is of no consequence for CF apps behind Gorouter.