Use top-level interface of session repository

[JohnNiang]

What type of PR is this?

/kind improvement
/area core
/milestone 2.17.x

What this PR does / why we need it:

Use top-level interface org.springframework.session.ReactiveFindByIndexNameSessionRepository for being compatible with other session repositories.

Which issue(s) this PR fixes:

Fixes #6159

Does this PR introduce a user-facing change?

None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.93%. Comparing base (bc10336) to head (7e30b71).
Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6160      +/-   ##
============================================
+ Coverage     54.51%   56.93%   +2.42%     
- Complexity     3523     3706     +183     
============================================
  Files           646      652       +6     
  Lines         21862    22038     +176     
  Branches       1528     1537       +9     
============================================
+ Hits          11917    12547     +630     
+ Misses         9328     8883     -445     
+ Partials        617      608       -9     

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[guqing]

/lgtm

[f2c-ci-robot]

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