Remove JobsCursor.next.

[bdice]

Description

Removes the long-deprecated JobsCursor.next method (deprecated since before 1.0).

Motivation and Context

signac 2.0 API cleanup.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[mikemhenry]

I think this is the oldest thing we have cut lately and I'm just waiting for this to show up on slack.
https://xkcd.com/1172/

[codecov]

Codecov Report

Merging #604 (03251c6) into next (a2cbd21) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             next     #604      +/-   ##
==========================================
- Coverage   85.77%   85.74%   -0.03%     
==========================================
  Files          52       52              
  Lines        5075     5066       -9     
  Branches      914      913       -1     
==========================================
- Hits         4353     4344       -9     
  Misses        522      522              
  Partials      200      200              

Impacted Files	Coverage Δ
signac/contrib/project.py	88.13% <ø> (-0.17%)	⬇️

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