Revert Rust 1.68.0 upgrade

[ilya-bobyr]

Problem

The nightly upgrade is breaking the geyser_plugin_manager. We will probably need more investigation before we can move forward with it.

Examples of failed CI runs:
https://buildkite.com/solana-labs/solana/builds/92624#0187160f-ec5e-40ae-84b8-a1859f44986a
https://buildkite.com/solana-labs/solana/builds/92627#0187161c-4431-4f62-945a-5e07869b3163
https://buildkite.com/solana-labs/solana/builds/92619#018715aa-c516-4f5f-886c-dd2f9628ec73

The CI log says

ci/intercept.sh: command failed; please see ./intercepted-console-2023y03m24d22h15m46s071144271ns.log in artifacts

In this log file I see:

+ exec cargo +nightly-2023-02-08 test --jobs 48 --target-dir target/cov --lib --all --exclude solana-local-cluster -- --nocapture
    Finished test [unoptimized + debuginfo] target(s) in 0.62s
warning: the following packages contain code that will be rejected by a future version of Rust: fs_extra v1.2.0
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`
     Running unittests src/lib.rs (target/cov/debug/deps/solana_account_decoder-85cdc6863184cf35)
[...]
error: test failed, to rerun pass `-p solana-geyser-plugin-manager --lib`

Caused by:
  process didn't exit successfully: `/solana/target/cov/debug/deps/solana_geyser_plugin_manager-9fbd116de18ee1f8 --nocapture` (signal: 11, SIGSEGV: invalid memory reference)

Unfortunately, I can not reproduce it locally. The same command as the one CI is running:

./cargo +nightly --target-dir target/cov --lib --all --exclude solana-local-cluster -- --nocapture

succeeds on my machine.

Summary of Changes

Just revert the upgrade for now.

[brooksprumo]

Lgtm. Thanks for handling this.

[codecov]

Codecov Report

Merging #30897 (fbfee9b) into master (24be850) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           master   #30897     +/-   ##
=========================================
- Coverage    81.5%    81.5%   -0.1%     
=========================================
  Files         726      726             
  Lines      204914   204907      -7     
=========================================
- Hits       167065   167039     -26     
- Misses      37849    37868     +19