Switch to BurntSushi/toml and error out on unknown toml keys

[ondrejbudai]

This pull request includes two changes. First, it switches from pelletier/go-toml to BurntSushi/toml in the bib module. This change removes a dependency and aligns with the use of BurntSushi elsewhere in the codebase. Second, it adds error handling for unknown toml keys in the buildconfig module. This change improves clarity and prevents confusion when using older versions of bib with recent customizations.

[mvo5]

Thank you!

[mvo5]

I had to rebase because of conflicts.

[bcl]

Looks like this broke a test:

=== RUN   TestReadUserConfigErrorWrongFormat
    config_test.go:100: 
                Error Trace:    /home/bcl/Red_Hat/projs/bootc-image-builder/bib/internal/buildconfig/config_test.go:100
                Error:          Error "cannot decode \"/tmp/TestReadUserConfigErrorWrongFormat3091236278/001/config.toml\": toml: line 1: expected '.' or '=', but got '{' instead" does not contain "parsing error"
                Test:           TestReadUserConfigErrorWrongFormat
--- FAIL: TestReadUserConfigErrorWrongFormat (0.00s)