fix(bump): accept lowercase values for bump_type config

[orhun]

Description

bump_type was intended to be used lowercase and that's what we suggest in the docs. However the implementation says otherwise.

Motivation and Context

Basically fixes that issue.

See #1066 (comment)

How Has This Been Tested?

Not tested, I believe in myself.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.47%. Comparing base (bf50336) to head (a0abacc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1101   +/-   ##
=======================================
  Coverage   41.47%   41.47%           
=======================================
  Files          21       21           
  Lines        1811     1811           
=======================================
  Hits          751      751           
  Misses       1060     1060           

Flag	Coverage Δ
unit-tests	41.47% <ø> (ø)

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