Maintanence updates

[benmaddison]

• bump msrv and update manifests
• add devShells
• add rust-src to toolchains
• Maintenance updates

[codecov]

Codecov Report

❌ Patch coverage is 56.45161% with 81 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.97%. Comparing base (2ee917e) to head (1d23501).

Files with missing lines	Patch %	Lines
netconf/src/message/rpc/error.rs	54.16%	22 Missing ⚠️
.../message/rpc/operation/junos/load_configuration.rs	0.00%	14 Missing ⚠️
junos-agent/src/policies/fetch.rs	65.78%	13 Missing ⚠️
...essage/rpc/operation/junos/commit_configuration.rs	0.00%	6 Missing ⚠️
junos-agent/src/netconf/pem.rs	0.00%	5 Missing ⚠️
netconf/src/message/rpc/mod.rs	61.53%	5 Missing ⚠️
netconf/src/message/rpc/operation/mod.rs	66.66%	4 Missing ⚠️
.../message/rpc/operation/junos/open_configuration.rs	0.00%	3 Missing ⚠️
netconf/src/capabilities.rs	60.00%	2 Missing ⚠️
netconf/src/message/rpc/operation/junos/mod.rs	0.00%	2 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   45.23%   49.97%   +4.74%     
==========================================
  Files          43       42       -1     
  Lines        3210     3852     +642     
==========================================
+ Hits         1452     1925     +473     
- Misses       1758     1927     +169     

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