Remove dep-versions-bump and install-requirement workflows

[freakboy3742]

With the move to dependency groups, there's no longer any need for the install-requirement workflow. All existing uses have been replaced with a direct pip install --group ... all.

The deps-version-bump script and workflow is also redundant now; historically, it was used to:

1. Bump the build system requirements in pyproject.toml, and
2. Update python versions pinned in tox.ini.

Dependabot is able to independently manage (1). With the move to dependency groups, there aren't any explicitly pinned versions in tox.ini any more - all versions are in dependency groups in pyproject.toml, which dependabot is able to manage.

All the references to the install-requirement workflow have been removed, except for one remaining reference in an archived repo, and the code this PR removes.

The remaining references to dep-version-bump have all been removed in submitted PRs:
Refs beeware/briefcase#2542
Refs beeware/toga#3856
Refs beeware/beeware#597
Refs beeware/rubicon-objc#666
Refs beeware/setuptools_dynamic_dependencies#56

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct