Changes

fix(tikv/rust-rocksdb): migrate CI from CentOS 7 to Rocky Linux 8 (#4564)

## Problem

`openssl-sys` v0.9.114 (released ~April 2026) dropped support for
OpenSSL < 1.1.0. The current CentOS 7 builder image installs
`openssl-devel` from the CentOS 7 repos, which provides OpenSSL
**1.0.2k** — now rejected at compile time with:

> "This crate is only compatible with OpenSSL (version 1.1.0, 1.1.1,
3.x, or 4.x), but a different version of OpenSSL was found."

This breaks the `encryption` feature build for **all PRs** targeting
`tikv/rust-rocksdb` (confirmed also broken on PR #849, an unrelated
change).

Upstream issue: tikv/rust-rocksdb#850

## Fix

Switch all three jobs (`pull-rust-rocksdb-lint`,
`pull-rust-rocksdb-test-x86`, `pull-rust-rocksdb-test-arm`) to the same
Rocky Linux 8 builder image that `tikv/tikv` latest presubmits already
use (`v2026.3.22-3-g892e449`).

Rocky Linux 8 ships OpenSSL 1.1.1 and a modern GCC system-wide, so:
- `yum install -y openssl-devel` is removed (pre-installed in the image)
- The `devtoolset-8` source block is removed (system GCC is sufficient)
- The CentOS-specific `llvm-toolset-7.0` SCL install is replaced with
`dnf install -y clang-tools-extra`

## Precedent

`tikv/tikv` latest presubmits already run on this image (see PR #4383).

Signed-off-by: Yishuai Li <yishuai.li@pingcap.com>
 (commit: 0905ad8)