Remove v2 + query for relation types

[ronenh]

In addition to removing utilities that support the v2 directory, this PR adds a new Cache.AssignableRelations() function that searches the model for relation types that can exist between an object type and a subject type (optionally with subject relations).

For example:

# Get relation types that can have a 'group' as the object and a 'user' as subject.
cache.AssignableRelations("group", "user")  )  # returns ["member"]

# Get relation types that can have 'tenant' as the object and `machine_account#user` as the subject.
cache.AssignableRelations("tenant", "machine_account", "user")  # returns ["decision-log-reader", 
                                                                #          "directory-client-reader", ...]

[coveralls]

Pull Request Test Coverage Report for Build 11577520789

Details

• 21 of 34 (61.76%) changed or added relevant lines in 2 files are covered.
• 36 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+5.1%) to 65.92%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
paging/slice.go	0	2	0.0%
cache/cache.go	21	32	65.63%

Files with Coverage Reduction	New Missed Lines	%
paging/cursor.go	13	0.0%
paging/slice.go	23	0.0%

Totals
Change from base Build 11001328867:	5.1%
Covered Lines:	2559
Relevant Lines:	3882

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💛 - Coveralls

[gertd]

See note about using golang 1.23 to compile and the associated bump of the linter to 1.61.0