feat: add support for cdx 1.6

[CBeck-96]

Adds the offical 1.6 schema as well as a custom version to the tool.

I removed the entire "licensechoice" section in the custom schema, like in 1.5, since the new implementation leads to misleading error messages.

[github-actions[bot]]

Coverage Report •

File	Stmts	Miss	Cover	Missing
TOTAL	1688	71	95%

report-only-changed-files is enabled. No files were changed during this commit :)

Tests	Skipped	Failures	Errors	Time
351	2 :zzz:	0 :x:	0 :fire:	10.891s :stopwatch:

[italvi]

Please also add the updated SPDX list

[mmarseu]

@CBeck-96 please take a look at #157. We decided to remove the not required files, e.g. the xml-schemata. And also add integration-tests for 1.6 as done there for 1.2 to 1.5

Since these required changes were on my head, I went ahead and implemented them. Integration tests are done and the non-JSON-schema files are gone.