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%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[bearomorphism]

@Lee-W@noirbizarre please review

[bearomorphism]

We might need to come up with another release strategy

We may prioritize this. Clarifying which commit new branches should be based on would make it much easier for new contributors, and it should also help reduce merge conflicts and PRs being accidentally closed due to test branch deletion.

[Lee-W]

One nit. Close to merge

[Lee-W]

We might need to come up with another release strategy

We may prioritize this. Clarifying which commit new branches should be based on would make it much easier for new contributors, and it should also help reduce merge conflicts and PRs being accidentally closed due to test branch deletion.

Generally feat goes to next minor release and others goes to next patch release. docs, tests can go the master directly