chore: Deprecate kyma-cli v2

[pPrecel]

Description

Changes proposed in this pull request:

• add a deprecation note at the beginning of the main README.md file
• add adeprecation note at the beginning of the kyma-cli main.go file

Related issue(s)

• https://github.com/kyma-project/cli/issues/2051
• https://github.com/kyma-project/cli/pull/2095