Compatibility with 1.75

[mxgrey]

Some recent updates to this repo have led to CI failing in rclrs for the minimal Rust version 1.75.

This PR aims to fix it via the following:

• Use a compatible resolver for the workspace Cargo.toml (2 instead of 3)
• Add a CI workflow that verifies this repo is working for both 1.75 and the latest stable
• Remove the Cargo.lock files so they don't create confusion for older versions of Rust

[mxgrey]

Apparently we were never running the tests for this package with the serde feature enabled, because we had some invalid test cases which are now fixed in this PR.

[jhdcs]

These changes look good to me, though I'd feel more comfortable if a second maintainer could have a look since I'm not the most experienced with workflows

[francocipollone]

Hey sorry for the intrusion. I was taking a look at ros2_rust's examples and following the instructions for setting up the workspace led me to this error (resolver 3 not available for 1.75).
Going in with this PR would be great so as to build the workspace from scratch without issues, especially for newcomers.

[luca-della-vedova]

Not a maintainer but anything that adds CI and proves to build correctly sounds like a good idea and upstream development on rclrs is effectively blocked until this repo works correctly, so even if there are any issues I'd suggest merging this first and fixing them in followups.

The only possible error surface is really the code in string.rs that looks sensible to me.

[maspe36]

Agreed, sorry for the delay on this PR everybody. I did screw up during the merge though and didn't squash the commits. Apologies (again) all

I updated the repo settings so that won't happen again