Warn user is using deprecated default source (repository reference instead of default = true)

[noahd1]

We recently changed the default source for newly generated qlty.toml files from:

[[source]]
name = "default"
repository = "https://github.com/qltysh/qlty"
branch = "main"

to:

[[source]]
name = "default"
default = "true"

This was so that the plugins referenced, by default, are guaranteed to be compatible with the binary used, because the plugins are compiled into the binary.

This works for newly generated qlty.toml files, but old generated ones in the wild should, ideally, be updated. They will continue to work as is, and a repository source is still a valid way to load sources, however, it is inherently more susceptible to incompatability issues than default = true.

This PR adds a warning if using the old-style default source.

Warning looks like this currently:

[qltysh]

The code coverage on the diff in this pull request is 100.0%

Drilldown

Path	File Coverage Δ
qlty-check/src/tool/github.rs	0.1
qlty-config/src/library.rs	0.7
qlty-check/src/parser/biome.rs	0.1
qlty-check/src/parser/trivy_sarif.rs	0.7
qlty-coverage/src/formats.rs	0.8
qlty-check/src/parser/phpstan.rs	0.8
qlty-check/src/parser/sqlfluff.rs	0.2
qlty-check/src/parser/knip.rs	-0.1
qlty-coverage/src/publish/settings.rs	0.0
qlty-check/src/parser/markdownlint.rs	1.0
qlty-config/src/sources/git_source.rs	35.5
qlty-check/src/parser/php_codesniffer.rs	0.6
qlty-check/src/parser/sarif.rs	0.1
qlty-check/src/parser/radarlint.rs	0.7
qlty-config/src/workspace.rs	0.4
qlty-coverage/src/transform/settings.rs	0.0
qlty-analysis/src/code/file.rs	3.3
qlty-check/src/parser/rubocop.rs	-0.2
qlty-check/src/parser/golangci_lint.rs	0.3
qlty-coverage/src/parser/jacoco.rs	-1.3
qlty-check/src/parser/ripgrep.rs	0.6
qlty-check/src/parser/tsc.rs	1.1
qlty-check/src/parser/coffeelint.rs	0.7
qlty-check/src/parser/stylelint.rs	0.7
qlty-check/src/parser/actionlint.rs	1.0
qlty-check/src/parser/bandit.rs	0.7
qlty-check/src/parser/eslint.rs	0.2
qlty-config/src/config.rs	9.9
qlty-smells/src/duplication/code.rs	1.3
qlty-check/src/parser/ruff.rs	0.4
qlty-analysis/src/workspace_entries/target_mode.rs	33.3
qlty-check/src/parser/clippy.rs	0.5
qlty-check/src/parser/trufflehog.rs	1.3

[qltysh]

All good ✅

[noahd1]

Addresssed comments / merged in main to get tests passing.