chore: rename agent-js to icp-js-core

[ilbertt]

Description

Remove or replace the occurrences of agent-js with icp-js-core. Once this is merged and the patch version for the packages is released, I will rename the repo on GitHub.

Checklist:

• My changes follow the guidelines in CONTRIBUTING.md.
• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[github-actions]

size-limit report 📦

Path	Size
@dfinity/agent	50.54 KB (0%)
@dfinity/assets	55.45 KB (0%)
@dfinity/auth-client	21.15 KB (0%)
@dfinity/candid	13.32 KB (0%)
@icp-sdk/core	91 B (0%)
@dfinity/identity	20.59 KB (0%)
@dfinity/identity-secp256k1	33.88 KB (0%)
@dfinity/principal	4.44 KB (0%)