Stop recommending mamba in installation guides and error messages

[utkarshalpha]

Description of proposed changes

• Replace Mamba commands with Conda commands in the six files identified in the issue.
• Collapse duplicated Mamba/Conda tab sets into shorter, single-path Conda instructions.
• Update optional-dependency error messages and the GMT release checklist to recommend Conda.
• Keep historical changelog references unchanged.

Conda has used the libmamba solver by default since 2023, so a separate Mamba path no longer provides enough benefit to justify duplicated instructions.

Fixes #4763

Preview:

Validation

• Repository pre-commit hooks on all six changed files
• Ruff check and format check on the modified Python modules
• Codespell on all changed files
• git diff --check
• Verified zero Mamba references remain in the six issue targets

Guidelines

• General Guidelines for Pull Request
• Guidelines for Contributing Documentation
• Guidelines for Contributing Code

[seisman]

Looks good. Thank you. I'll wait for another 24 hours to see if other maintainers have any additional comments.