bugfix: purge sessions on simplesaml upgrade

[matthewhilton]

Background

• Further testing of simplesaml update in 401 bump simplesamlphp 2.0.15 #907 has uncovered an edge case where if a user is logged in and the plugin is upgraded, their existing session gets stuck and the user cannot log in OR out. They need to purge their browser cache entirely.
  • This seems to be because the token format/deserialisation has changed between simplesaml versions, so it cannot deserialise the existing token and gets stuck

Changes

• Adds upgrade step for the version bumped in 401 bump simplesamlphp 2.0.15 #907 to purge existing sessions from the kvstore

Testing

• This requires saml setup locally, which I have.
• Using previous commit f7f1e99
• Log in as a user using saml
• Switch to f4a282b
• Try and log out as the user, it will throw an exception.
• Switch to this PR's branch, and run the site upgrade (with newly added upgrade step that deletes existing sessions)
• Confirm the user can now log in/out without error

I've done the above ^^ on my local environment and it worked

[matthewhilton]

^^ ci failing due to phpcs - unit tests passing

[brendanheywood]

my comment is not a blocker