fix(cargo): fix handling of x and x.y versions

[rami3l]

Changes

This PR changes the handling of cargo versions to be "caret by default".

Context

The Rustup project is now using what has been made available in #25983, which works well for an update from 1.2.0 to 1.4.1, for example, where the manifest is untouched under the rangeStrategy: update-lockfile mode.
However, when an update from 1.2 to 1.4 is detected, the manifest is still updated (see rust-lang/rustup#3569 (comment)).

The root cause of this issue seems to be that the current implementation of cargo versioning incorrectly assumes the following:

renovate/lib/modules/versioning/cargo/index.ts

Lines 24 to 26 in b69416c

	// NOTE: Partial versions like '1.2' don't get converted to '^1.2'
	// because isVersion('1.2') === false
	// In cargo and in npm 1.2 is equivalent to 1.2.* so it is correct behavior.

... however in cargo 1.2 is handled the same way as ^1.2, as documented in https://doc.rust-lang.org/stable/cargo/reference/specifying-dependencies.html#caret-requirements.

This PR combined with the previous #25983 should be able to address rust-lang/rustup#3541.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[rarkins]

@rami3l thanks for the elegant fix!

[renovate-release]

🎉 This PR is included in version 37.92.2 🎉

The release is available on:

• GitHub release
• 37.92.2

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