fix(saml): Allow overriding maxAuthenticationAge

[pieterza]

The default maxAuthenticationAge is 7200 in upstream spring module.
This change allows configuring the maxAuthenticationAge as per any other SAMLSecurityConfigProperties entry.

Example gate-local.yaml:

saml:
  maxAuthenticationAge: 604800

This made by possible by the following stale PR, and replaces it:
#550

[spinnakerbot]

The following commits need their title changed:

• 83acd58: Allow overriding maxAuthenticationAge

Please format your commit title into the form:

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[eaceaser]

@spinnakerbot cherry-pick 1.15

[spinnakerbot]

Cherry pick successful: #865