scripts: fix check for container version

[patrick-stephens]

The 2.2.0 release of Fluent Bit failed to update the container image version: #1252

https://github.com/fluent/fluent-bit/actions/runs/6810802273/job/18519927519#step:6:22

+ MAJOR_VERSION=2.2
+ grep -q 2.2.0 /home/runner/work/fluent-bit/fluent-bit/installation/docker.md
+ echo 'Found 2.2.0 already in the Docker docs so skipping update'
Found 2.2.0 already in the Docker docs so skipping update

This was resolved in #1268.

The reason for the failure is grep using the value passed as a regex rather than a fixed literal string:

$ grep -F "2.2.0" installation/docker.md
| 2.2.0-debug | x86_64, arm64v8, arm32v7 | Debug images |
| 2.2.0 | x86_64, arm64v8, arm32v7 | Release [v2.2.0](https://fluentbit.io/announcements/v2.2.0/) |
$ grep "2.2.0" installation/docker.md
| 2.2.0-debug | x86_64, arm64v8, arm32v7 | Debug images |
| 2.2.0 | x86_64, arm64v8, arm32v7 | Release [v2.2.0](https://fluentbit.io/announcements/v2.2.0/) |
[{"critical":{"identity":{"docker-reference":"index.docker.io/fluent/fluent-bit"},"image":{"docker-manifest-digest":"sha256:c740f90b07f42823d4ecf4d5e168f32ffb4b8bcd87bc41df8f5e3d14e8272903"},"type":"cosign container image signature"},"optional":{"release":"2.0.6","repo":"fluent/fluent-bit","workflow":"Release from staging"}}]

Notice in the second one we match the string 27290 as this is the 2.2.0 regex.