Update solargraph gem version

What does this MR do?

Updates solargraph gem version which includes various performance improvements and features: https://github.com/castwide/solargraph/blob/master/CHANGELOG.md

## 0.42.3 - June 14, 2021
- Require 'pathname' for Library

## 0.42.2 - June 14, 2021
- Improve download-core command output
- Ignore missing requests to client responses
- Add automatically required gems to YardMap
- Use closures to identify local variables

## 0.42.1 - June 11, 2021
- YardMap#change sets new directory (#445)

## 0.42.0 - June 11, 2021
- Improve YardMap efficiency
- Bench includes Workspace for cataloging
- Initialize confirms static features from options (#436)
- Enable simple repairs without incremental sync (#416)
- Discard unrecognized client responses
- Notify on use of closest match for core (#390)

## 0.41.2 - June 9, 2021
- Rescue InvalidOffset in async diagnosis
- Remove erroneous escaping from Hover

## 0.41.1 - May 31, 2021
- ApiMap handles required bundles (#443)

## 0.41.0 - May 30, 2021
- Chain constant at last double colon with more than two nested namespaces
- Fill Integer#times return type (#440)
- Validate included modules in type checks (#424)
- Faster language server initialization
  - Server response to initialize is near immediate
  - Workspace is mapped in a background thread
  - Supported clients report mapping progress
- Log RuboCop corrections at info level (#426)
- Allow configuring the version of RuboCop to require (#430)
- Fix source of diagnostic (#434)
- Fix file argument in RuboCop (#435)
- Config ignores directories with .rb extension (#423)

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)