Bump golang from 1.20.5-alpine3.17 to 1.21.5-alpine3.17

Bumps golang from 1.20.5-alpine3.17 to 1.21.5-alpine3.17.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20.5-alpine3.17 as builder
+FROM golang:1.21.5-alpine3.17 as builder
 
 WORKDIR /go/src/github.com/mxssl/tg-captcha-bot
 COPY . .