Bump Kani version to 0.61.0.

Github-generated release notes:

## What's Changed
* Fix CHANGELOG of 0.60.0 by @qinheping in
#3925
* Bump tests/perf/s2n-quic from `d88faa4` to `8670e83` by @dependabot in
#3928
* Update toolchain to 2025-03-04 by @qinheping in
#3927
* Install the right toolchain for HEAD and BASE checks in
`verify-std-check.yml` by @remi-delmas-3000 in
#3920
* Automatic cargo update to 2025-03-10 by @github-actions in
#3926
* Automatic toolchain upgrade to nightly-2025-03-05 by @github-actions
in #3929
* Upgrade toolchain to nightly-2025-03-07 by @tautschnig in
#3931
* Upgrade toolchain to nightly-2025-03-12 by @tautschnig in
#3933
* Automatic toolchain upgrade to nightly-2025-03-13 by @github-actions
in #3934
* Update CBMC dependency to 6.5.0 by @tautschnig in
#3936
* Automatic toolchain upgrade to nightly-2025-03-14 by @github-actions
in #3937
* Automatic toolchain upgrade to nightly-2025-03-15 by @github-actions
in #3938
* Automatic toolchain upgrade to nightly-2025-03-16 by @github-actions
in #3939
* Automatic toolchain upgrade to nightly-2025-03-17 by @github-actions
in #3940
* Automatic cargo update to 2025-03-17 by @github-actions in
#3941
* Autoharness: Don't panic on `_` argument and add `_autoharness` suffix
to GOTO files by @carolynzech in
#3942
* Implement `f16` and `f128` cases in `codegen_float_type` by
@carolynzech in #3943
* Support function implementations of known built-ins by @tautschnig in
#3945
* Autoharness: metadata improvements and enable standard library
application by @carolynzech in
#3948
* Autoharness: `--list` option by @carolynzech in
#3952
* Add support for anonymous nested statics by @carolynzech in
#3953
* Automatic cargo update to 2025-03-24 by @github-actions in
#3954
* Bump tests/perf/s2n-quic from `8670e83` to `324cf31` by @dependabot in
#3955
* Document behavior of checked_size_of_raw and is_inbounds by @rajath-mk
in #3956
* Upgrade toolchain to 2025-03-18 by @zhassan-aws in
#3959
* Remove unstable-features from code formatting script by @zhassan-aws
in #3962
* Remove CI job to update features/verify-rust-std by @tautschnig in
#3963
* Make is_inbounds public by @rajath-mk in
#3958
* Enable Kani to work with a stable toolchain by @zhassan-aws in
#3964
* Automatic cargo update to 2025-03-31 by @github-actions in
#3966
* Add support for struct field accessing in loop contracts by
@thanhnguyen-aws in #3970
* Bump tests/perf/s2n-quic from `324cf31` to `d0aff82` by @dependabot in
#3968
* Clarify `is_inbounds` docs by @carolynzech in
#3974
* Upgrade toolchain to 2025-04-01 by @carolynzech in
#3973
* Remove remaining `--enable-unstable` mentions by @carolynzech in
#3978
* Clean up unused dependencies by @zhassan-aws in
#3981
* Automatic toolchain upgrade to nightly-2025-04-02 by @github-actions
in #3983
* Update dependencies per `cargo-outdated` by @carolynzech in
#3982
* Fix `autoharness` termination test & print metadata in alphabetical
order by @carolynzech in
#3971
* Fix cargo invocations to only use `pkg_args` where appropriate by
@carolynzech in #3984


**Full Changelog**:
kani-0.60.0...kani-0.61.0

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.