Add support for connection_max_age parameter

What does this MR do?

gitlab-org/gitlab-runner!4537 (merged) added a new global parameter, connection_max_age, for limiting the maximum length of a TLS keepalive connection. This commit adds support for configuring this value.

Why was this MR needed?

It was not possible to change the default value of 15 minutes to something else.

What's the best way to test this MR?

Set values.yaml to 10m:

diff --git a/values.yaml b/values.yaml
index 82d51a9..576f6d7 100644
--- a/values.yaml
+++ b/values.yaml
@@ -121,7 +121,7 @@ checkInterval: 30
 ## Configure GitLab Runner's maximum connection age for TLS keepalive connections.
 ## ref https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-global-section
 ##
-# connectionMaxAge: "15m"
+connectionMaxAge: "10m"

 ## A custom bash script that will be executed prior to the invocation of the

Then run:

helm upgrade --install gitlab-runner --set gitlabUrl=https://gitlab.example.com,runnerRegistrationToken=glrt-REDACTED --set rbac.create=true .

Verify the setting:

$ grep max_age ~/.gitlab-runner/config.toml
connection_max_age = "10m0s"