Upgrade redis dependency to v9

[hgokhale]

We are having some dependency issues so wanted to upgrade redis dependency to V9

All tests are passing after this change

=== RUN   TestTokenStore

  Test redis token store 
    Test authorization code store ✔✔✔✔✔✔
    Test access token store ✔✔✔✔✔✔
    Test refresh token store ✔✔✔✔✔✔


18 total assertions

--- PASS: TestTokenStore (0.02s)
=== RUN   TestTokenStoreWithKeyNamespace

  Test redis token store 
    Test authorization code store ✔✔✔✔✔✔
    Test access token store ✔✔✔✔✔✔
    Test refresh token store ✔✔✔✔✔✔


36 total assertions

--- PASS: TestTokenStoreWithKeyNamespace (0.02s)
PASS
ok  	github.com/go-oauth2/redis/v4	(cached)

[hgokhale]

Created another PR from separate branch so closing this one - #18