Replace DS_PIPCOMPILE_REQUIREMENTS_FILE_NAME_PATTERN with DS_PIPCOMPILE_LOCKFILE_FILE_NAME_PATTERN
## Problem to solve
The dependency scanning analyzer currently exposes a CI/CD variable named
`DS_PIPCOMPILE_REQUIREMENTS_FILE_NAME_PATTERN` which is used to point at a
**lockfile** (a `requirements.txt` generated by `pip-compile`) so the analyzer
can scan it directly.
The current name is ambiguous because in the Python ecosystem `requirements.txt`
can be **either**:
- a **manifest** (loose, hand-authored constraints — input to `pip-compile`), or
- a **lockfile** (fully pinned output of `pip-compile`).
Today the variable name says `REQUIREMENTS`, which most Python users will read
as "manifest". This is the opposite of what the variable actually does.
The confusion was discussed in detail in
[the parent issue](https://gitlab.com/gitlab-org/gitlab/-/work_items/593859#note_3215486291).
## Proposal
Replace `DS_PIPCOMPILE_REQUIREMENTS_FILE_NAME_PATTERN` with
`DS_PIPCOMPILE_LOCKFILE_FILE_NAME_PATTERN` everywhere it is referenced, while
keeping the old name working as a deprecated variable to
avoid breaking existing pipelines.
Similarly, replace the corresponding spec:input `pipcompile_requirements_file_name_pattern` with `pipcompile_lockfile_file_name_pattern`.
### Naming rationale
| Variable | Points at | Used by |
|----------|-----------|---------|
| `DS_PIPCOMPILE_LOCKFILE_FILE_NAME_PATTERN` (new) | pip-compile **lockfile** (pinned `requirements.txt`) | DS analyzer (direct scan) and DR service (skip directory) |
| `DS_PIPCOMPILE_REQUIREMENTS_FILE_NAME_PATTERN` (deprecated alias) | same as above | kept as alias for backward compatibility |
This naming aligns with the convention being introduced for the future
manifest-side variable (`DS_PIP_MANIFEST_FILE_NAME_PATTERN`), so users get an
unambiguous "manifest vs lockfile" split as
[discussed in the issue](https://gitlab.com/gitlab-org/gitlab/-/work_items/593859#note_3217259587).
issue