Fix code scanning alerts

[jescalada]

GitHub's automatic security and quality scanning has identified a few potential issues in our code. Although a few of these have been published a long time ago, we should still figure out whether they're relevant and fix them up - if anything to improve our code quality.

Many of these are related, and easy to fix:

Type confusion in parsePush.ts:

https://github.com/finos/git-proxy/security/code-scanning/141
https://github.com/finos/git-proxy/security/code-scanning/140
https://github.com/finos/git-proxy/security/code-scanning/139
https://github.com/finos/git-proxy/security/code-scanning/138

Excessive workflow permissions

https://github.com/finos/git-proxy/security/code-scanning/143
https://github.com/finos/git-proxy/security/code-scanning/133
https://github.com/finos/git-proxy/security/code-scanning/132

Unpinned dependencies

https://github.com/finos/git-proxy/security/code-scanning/120
https://github.com/finos/git-proxy/security/code-scanning/113

The only one that isn't eay to fix is this one:
https://github.com/finos/git-proxy/security/code-scanning/64

Most vulnerabilities come from specific packages that don't get updated, the biggest culprit being docusaurus.

Describe the solution you'd like
We should fix these to improve GitProxy's code quality and inspire trust in potential adopters.

[kriswest]

Type confusion in parsePush.ts is the critical one and looks like all 4 issues are the same thing, which can be resolved by asserting that req.body is a string with typeof and throwing an error if not.

Regarding the last one - if many of the issues are coming from docusaurus dependencies I should be able to resolve that. I've recently done so on the FDC3 project, where I had to work out a few overrides (as docusaurus doesn't get a clean audit currently upstream): https://github.com/finos/FDC3/pull/1831/changes#diff-fae242fbf77a8a9d52625664bc36ea12316586f8a716b41137d897a2b7e3df76.

[jescalada]

It'd be awesome if you could fix up the docusaurus dependencies! It's been a while since we last tried fixing the ones marked as GHSA vulnerabilities. I remember I dealt with many of them last year and about ~20 were coming from docusaurus. I don't know where all the extra vulnerabilities are coming from, though...

I can open a PR to deal with the other code scanning alerts and non-docusaurus GHSA affected deps👍🏼

*Edited this for clarity