Bump CHANGELOG and VERSION for v3.51.0

[triarius]

I decided to mark all the signature features as [Experimental]. There's precedent:

agent/CHANGELOG.md

Line 789 in 24fbc05

* Experimental: Build & publish RPM ARM64 package for aarch64 [#1243](https://github.com/buildkite/agent/pull/1243) ([chloeruka](https://github.com/chloeruka)) [#1241](https://github.com/buildkite/agent/pull/1241) ([chloeruka](https://github.com/chloeruka))

v3.51.0 (2023-08-15)

Full Changelog

Added

• Add --strict-single-hooks #2268 (@DrJosh9000)
• Add missing 'an' in annotation help #2285 (@mdb)
• [Experimental] Verify step signatures #2210 (@moskyb)
• [Experimental] Pipeline Signing/Verification with JWS #2252 (@moskyb)
• [Experimental] Include plugins in command step signatures #2292 (@DrJosh9000)

Changed

• Make the agent send a SIGTERM (configurable) before a SIGKILL to subprocesses #2250 (@triarius)
• Limit job log length #2192 (@DrJosh9000)
• Refactor redactor into streaming replacer and use it to redact secrets #2277 (@DrJosh9000)
• Dependency upgrades #2278 #2274 #2271 #2272 #2275 #2266 (@dependabot[bot])

Fixed

• Fix fatal: bad object not detected from git fetch #2286 (@triarius)
• Fix scalar plugin parsing #2264 (@DrJosh9000)

Internal

• Reorganise step types among files #2267 (@DrJosh9000)
• Upload test coverage #2270 (@DrJosh9000)
• Remove unwrapping in error Is methods #2269 (@triarius)
• Use capacity hint in concat #2288 (@DrJosh9000)
• Add ordered.Unmarshal, and use it in pipeline parsing #2279 (@DrJosh9000)
• Create a setup method for config and logger to reduce boilerplate #2281 (@triarius)
• Add retry for publishing RPMs #2280 (@triarius)
• Fix data race in testAgentEndpoint #2265 (@DrJosh9000)
• Fix missing "fmt" import #2287 (@DrJosh9000)