Dev/update version and notes

[chifflier]

Misc changes to prepare first beta release (which will be 0.19.0-beta.1):

• Fix version number in CHANGELOG and UPGRADING files (closes #222)
• Update version to 0.19.0-beta.1
• chore(cargo): update dependencies time and thiserror
  • note that time depends on time-core which unfortunately requires rust 1.81, which explains the next commits
  • compatibility with our current MSRV is still possible by using a --precise flag with cargo update
• Clarify MSRV policy in README
• switch to cargo rdme, add CI checks and update links
  • this helps maintaining src/lib.rs and README.md in sync