fix(bump): remove NotAllowed related to --get-next option, and other related refactoring

[bearomorphism]

Changes

1. Remove unreachable logic, such as setting dry_run = True while we already have get_next = True. The latter condition raises a GetNextExit() exception in the original code, so no effect if we set dry_run to True. Moreover, the original code is self.dry_run = True which is ineffective since we never use the member variable.
2. Simplify things.
3. Rename the variable get_next to next_version_to_stdout. The variable name get_next is very unclear and hard to guess what it will do. I think we can consider renaming the option in the next major release.
4. (relate Not possible to use --get-next when changelog is enabled #1640) Replace NotAllowed related to --get-next option with warnings.
5. Remove GetNextExit exception because we already have DryRunExit. --get-next is just --dry-run but doing less things IMO.

[bearomorphism]

Is the branck v4-10-1 broken?

[woile]

LGTM

[Lee-W]

I just update the v4-10-1 branch. Could you please rebase from it again? We might need to come up with another release strategy, but this is what we have for the time being

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v4-10-1@9f3ec86). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
commitizen/commands/bump.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             v4-10-1    #1645   +/-   ##
==========================================
  Coverage           ?   98.64%           
==========================================
  Files              ?       60           
  Lines              ?     2651           
  Branches           ?        0           
==========================================
  Hits               ?     2615           
  Misses             ?       36           
  Partials           ?        0           

Flag	Coverage Δ
unittests	98.64% <87.50%> (?)

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[bearomorphism]

@Lee-W @noirbizarre please review