[YUNIKORN-2671] Convert Allocation releases field to singular

[craigcondit]

What is this PR for?

The releases slice in the Allocation object is now redundant since multiple allocations for a single release are no longer allowed. Convert the releases slice into a single release reference instead.

What type of PR is it?

• - Bug Fix
• - Improvement
• - Feature
• - Documentation
• - Hot Fix
• - Refactoring

Todos

• - Task

What is the Jira issue?

https://issues.apache.org/jira/browse/YUNIKORN-2671

How should this be tested?

Unit tests updated as needed.

Screenshots (if appropriate)

Questions:

• - The licenses files need update.
• - There is breaking changes for older versions.
• - It needs documentation.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.66%. Comparing base (d515fe0) to head (1a3640e).

Files	Patch %	Lines
pkg/scheduler/context.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #890      +/-   ##
==========================================
+ Coverage   77.63%   77.66%   +0.03%     
==========================================
  Files          97       97              
  Lines       12119    12104      -15     
==========================================
- Hits         9408     9401       -7     
+ Misses       2388     2379       -9     
- Partials      323      324       +1     

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[pbacsko]

LGTM