Update abseil to the latest LTS.

[bstaletic]

This should be enough to fix ycm-core/YouCompleteMe#4212

@Jaywalker, would you mind double checking my choice of abseil commit?

@puremourning Abseil's cmake is warning about this:

https://github.com/abseil/abseil-cpp/blob/lts_2023_08_02/CMakeLists.txt#L73-L78

We should figure out how to set that.

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This change is 

[codecov[bot]]

Codecov Report

Merging #1725 (4d1accc) into master (b34008a) will not change coverage. Report is 2 commits behind head on master. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1725 +/- ## ======================================= Coverage 95.47% 95.47% ======================================= Files 83 83 Lines 8177 8177 Branches 163 163 ======================================= Hits 7807 7807 Misses 320 320 Partials 50 50 ```

[Jaywalker]

An eyes-only review shows it has the patch necessary for the OpenBSD/FreeBSD fix. I'll do full build of it on my OpenBSD machine in a little bit and confirm it works, but I fully expect it will!

[mergify[bot]]

Thanks for sending a PR!

[mergify[bot]]

Thanks for sending a PR!