Implemented: Change the flag --project-manifest to --manifest-path #1333

[amartyadav]

1. crates/release_plz/src/args/release.rs AND crates/release_plz/src/args/update.rs

• Commented out (line 20 release.rs, line 22 update.rs):

  #[arg(long, value_parser = PathBufValueParser::new())] for project_manifest: Option<PathBuf> to still keep the project_manifest flag as requested in issue #1333 "Keep the old flag for cli retro-compatibility".

• Added (line 23 - 27 in release.rs, line 25 - 29 in update.rs):

  #[arg(long, value_parser = PathBufValueParser::new())] manifest_path: Option<PathBuf> with the same '--help' message as the old flag.

• Changed (line 139 release.rs, line 102 update.rs) -> from self.project_manifest to self.manifest_path

• Added (line 205 release.rs, line 243 update.rs) -> to include manifest_path: None,

2. website/docs/github/quickstart.md

• Renamed (line 217, line 245) -> the Github Action input parameter from project_manifest to manifest_path

Additional Notes:

• All tests passing (including Docker tests)
• Code formatted
• Followed all steps in Contributing.MD