Use SPDX identifiers for unknown/undetected license

[VannTen]

Problem statement

We currently have an ad-hoc 'UNDETECTED' license for case where we don't detect
the license. This leads to some special casing (see the linked PR for example)
which increases our code complexity.

High-level Goals

Instead of UNDETECTED, use the standard SPDX identifiers.

Proposal description

Since we use SPDX identifiers, we should use the standard they have
for that:

• https://scancode-licensedb.aboutcode.org/unknown-spdx.html
• https://scancode-licensedb.aboutcode.org/unknown.html
• https://scancode-licensedb.aboutcode.org/no-license.html

Alternatives

N/A

Additional context

#2705 (comment)

Acceptance Criteria

• Consumers and producers of the license data use the correct SPDX
  identifiers and avoid special-casing
• 'UNDETECTED' is replace by 'no-license' in the DB

/triage needs-information
(as I don't know where the code producing and consuming this is right now)
/sig stack-guidance

[sesheta]

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If there is new input, reopen with /reopen.