INSTALL.md: fix Go version and drop obsolete root-user INSERT (#722)

[Amaan0907]

Fixes #722.

Changes

• Go prerequisite: 1.24.2+ → 1.25+, matching go.mod.
• Root user INSERT removed: the "First run" SQL used MySQL's UNIX_TIMESTAMP(),
  which fails on PostgreSQL. The step is also no longer needed: EnsureRootUser
  (api/iam/service/bootstrap.go, called from cmd/airavata-server/main.go) creates
  the root row on startup, and its doc comment says it replaces this manual INSERT.
  Running the INSERT now would hit a duplicate-key error, so I removed it instead of
  porting it to PostgreSQL.
• Updated the matching Troubleshooting row.

Docs-only change.

[DImuthuUpe]

@Amaan0907 did you run the project with suggested go version?

[Amaan0907]

@DImuthuUpe
Yes , tested on Windows 11 with Go 1.25.5, against the compose PostgreSQL 17:

• go build ./cmd/airavata-server and go vet ./... pass.
• The server starts, auto-migrates and prints the root token; GET /health returns
  {"status":"UP"}, and creating a cluster with the root token returns 201.
• On startup the server creates the root row in users itself (EnsureRootUser).
• The old INSTALL.md INSERT fails with function unix_timestamp() does not exist.
  A PostgreSQL port of it, run after the server has started, fails with
  duplicate key value violates unique constraint "users_pkey", so the step is
  redundant and the PR removes it.

go test ./... passes except TestBuildSlurmScript (internal/orchestration). It
fails the same way on a clean checkout of master (93d1bd4): the test expects
<id>.stdout / <id>.stderr, but slurm.go writes stdout.txt / stderr.txt.
That's unrelated to this docs change; happy to open a separate issue/PR for it.