Drop support for Vault 1.6-1.10

[briantist]

Breaking changes

• Drop unsupported Vault versions 1.6 through 1.10 in CI (https://endoflife.date/hashicorp-vault)
  • 1.6 and 1.7 remain tested as enterprise versions, mostly because we haven't been able to properly test enterprise versions without a license past those
  • unsupported versions still remain, we will probably become more aggressive about dropping them in the future
  • while dropping these from CI doesn't mean that hvac won't work against these versions, it does mean we don't test against them and don't enforce new or edited code to be compatible with them

[briantist]

It seems there's some change in the behavior of kubernetes support in Vault 1.15.x and a test is not getting the result that is expected. I'm going to make this PR about dropping older versions for 2.0.0, and adding 1.15 will have to be done later when there's time to investigate the kubernetes issues. Adding a new Vault version won't be a breaking change and can be done in a minor version.

• Add Vault 1.15.x to integration tests #1075

[codecov]

Codecov Report

Merging #1074 (5062c27) into main (b8322ca) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1074   +/-   ##
=======================================
  Coverage   86.99%   86.99%           
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  Files          64       64           
  Lines        3115     3115           
=======================================
  Hits         2710     2710           
  Misses        405      405